I think this is the
link which has the most info on Tomba kits - it is mostly by Tomislav (Tromic) who designs the Tomba kits and it basically chronicles the development of the different models and as such also what is available and how they work. It is quite long and you might be able to start halfway through(?). A lot of it is intermediary design drafts and thoughts from Tomi with long-time Darksiders giving feedback and as such it also serves a great educational purpose.
Both Tomi and his son, Marko, who actually makes the kits, and other experienced members have been extremely helpful in answering a lot of my questions on the same subject - I am looking for a kit as well these days and want to make as an informed decision as possible:
http://forums.deeperblue.com/pneumatic-spearguns/96423-choosing-spear-dry-barrel-gun.html
Basically, from what I have been able to gather so far, all the manufacturers make kits for the Cressi guns (and most other brands) and in my opinion the biggest decision you have to make is whether you want the shooting line tied to a slider or to the tip of the spear. If you don't want a slider then you have at least three options (Tomba, STC and Seatec Evo-Air). If you do want a slider, then afaik, the Seatec Evo-Air is out of the picture. BTW, Tomba is the cheapest (Euros 60-65), STC quite a bit more (85-100) and Evo-Air is in between (75, with discount). Both Evo-Air and Tomba runs regular, cheap O-rings whereas STC uses more specialized seals (Euros 3.50/Ea). I really don't see this as much of an issue though. I believe they will all last a long time.
In regards to shaft thicknesses, Tomba is custom made to one size (or two that are very, very close to each other) and you have to change app. half of the parts to go with another size. STC is more modular and you'll have to change fewer, cheaper parts whereas Evo-Air can run shafts from 6.5mm to 7.5mm without changing anything (perhaps just the O-ring). This is not an issue at all if you know you are going with one shaft thickness, which I think the vast majority of people do anyways. I am just undecided and may want to keep more options open. So, if you want a kit that can run eg. two different shaft sizes then the price difference between STC and Tomba is neglible (both becomes between 100-110) and Evo-Air becomes the cheaper option (if you can find the 75 discount option, normally 100). One last difference between the kits worth mentioning, with Evo-Air, you have to screw in the head of the muzzle before loading and loosen it before shooting each time. If you forget to loosen it, you should be fine, you'll just have a bit more friction and a less powerful shot. With the others, you just load and fire.
Without having tried any of these options, I really like that Marko and Tomi (Tomba) are on this forum as much as they are and I have never come across a single post from anyone having any issues with their products. I have spent a long time reading about it all and I have only seen positive feedback. I think as a rule, people are more driven to post when they have bad experiences so I take this as a very strong indicator that Tomba just plain works! Also, I am not sure about those extra steps needed for the Evo-Air to function at its fullest, but maybe that's a non-issue and just becomes routine very fast.
I don't know if this is fair to ask, but perhaps we should keep responses to this on
the thread, I have already started as it is very much along the same lines? It would be great to have it all compiled in fewer threads for the future converters
. I will copy this post to there as well.
(Just tell me off, if I am wrong. I think I will also ask a mod to change the name of the thread I have linked to since it is now mostly about the dry barrel kits and not only about spears any longer.)
Best,
David