I got myself a 2004 model of a Triumph Sprint ST 955i this year. Although I was initially trying to get the bike with the fitted genuine Triumph pannier kit, I failed to get one for a good price and decided to go for one without it.
I thought it should be possible to fit it separately and luckily it is... although my way was possibly not straight forward. Anyway I wanted to tell you what I came up with and maybe spare you a few problems I came across.
Since the 2004 model of the Sprint ST 955i was the last of this series, it is rather hard to find the original panniers in the right colors. Even if you find them on ebay you end up with problems finding the fitting kit and instructions. At least I found it difficult.
Additionally, if you want to fit the genuine panniers, the exhaust needs to be lowered. This does not look very impressive with the panniers of. Of course if you have the genuine panniers, most people probably keep them on at all times.
There are a few after market kits on the market. I found it possible to order from Hepco & Becker and SW-Motech. Both manufacturers from Germany promising some quality but are not really cheap to get.
At the end I decided to get the SW-Motech kit, as I came across several rider bloggs having this kit installed successfully. The advantage of the SW-Motech kit versus the Hepco & Becker kit is the possibility to remove it very easy and quick, keeping the beautiful line of the Triumph undisturbed. Furthermore, you can get an adapter kit for almost every type of pannier (Givi, Hepco & Becker, Krauser, etc). A friend had an older set of Krauser K3 panniers (40liter) left over and suggested to use them, which also saved me some serious expenses for new ones.
The kit is available from several online sources, but I finally ordered it from www.touringbiker.com as I met them at a BMF show. Throughout the ordering and installation they have been very helpful and always provided feedback when needed. They also helped overcoming some of the issues I came across.
Price
The kit costs 168GBP + 35GBP for the Krauser adapter kit and some delivery costs. So overall it was about 200GBP.
Installation
It is rather difficult to find proper installation instructions and the provided leaflet was very rudimentary and left out several information. I do not have a garage or many tools and arranged the installation with a friend who had both. However, as I hoped to have everything needed in advance we were a bit taken aback with too little information and some misfitting parts. So the first attempt failed and only after the 3rd session all parts were properly in place.
I felt that clear photos from the installation would have helped a lot and therefore provide them here.

During this first installation ordeal I also noticed that both plastic body stabilizers fixed under the rack, were broken. I "fixed" them roughly with tape as you can see. This was something I did not notice when I bought the bike, but I don't think it is an uncommon problem and the body seems to hold so far.





That maybe a common problem as the indicator nuts are constantely exposed to dirt and water. Of course if you do not have replacements available you can't really ride the bike directly after... so better be prepared and check first if you can remove them without hassle.

After the indicator removal you can easily go on to fix the last to fixing points for the side-pannier holder.


Later on we added two washers under each of the two spacers in order to lift the anchor point a bit higher and provide more space to the body.

As you can see here, you can that mark the holes that need to be drilled to fix it to the license plate panel.



Then just fix this last part with the
provided screws, attach the (new) indicators onto it and simply attach the Krauser adapter kit and K-wings (not provided in the adapter kit! They should come with the panniers, I think?! At least I got them with the panniers.)

At the end it looks like here on the left.
Quite nice actually... with and without the Krauser panniers.
Problems
As I mentioned initially, there were two more issues besides the indicator removal hazard (that had nothing to do with the kit).
Firstly...



With the original fixation of the upper rear anchor point ( as shown above) the quick-lock pins touched the body. This has pierced the plastic and will most likely destroy the body or at least the paint.
I solved this by adding two washers to lift this fixation point a bit. As the two washers were not providing enough space, we also cut the tip of the quick-lock pin. Both finally provided enough space between the body and the pin. However, if you do this you can probably not return the kit and should consider possible corrosion.
(I just didn't want to go through that hassle of returning it and not having a pannier holder anyway... so I took the risk.)
Second problem:
The Krauser adapter kit was not fitting properly onto the base plate. As you can see to the left, we had problems to get the provided screws into the indicated holes. We had to extend the holes by drilling them up, until they finally fitted. Obviously this might also lead to corrosion in the long run. One might consider to paint or grease it to slow this process down.
Summary
With some modifications the kit can be fitted and is easily removable. It looks rather good in my eyes and feels very sturdy if everything is fitted.
I haven't yet done any proper travel tests, but will let you know if I do. It seems others (search forums) have used it for extensive time periods and it holds up to its promises.
Hope this helps.
Enjoy!