It's a conversion kit, and doesn't really void the warranty since you keep your old receiver halves and can convert back at any time, but I couldn't find a better place to post it. I got the magfeed kit and I like it a lot. The same kit works for both 98 and BT-4, and I had a BT4 that I converted.
As a prologue-note: I also tried the RAP4 Aluminum Powertube and Front Bolt, but the gun wouldn't even recock and the achieved velocity was barely enough to lob a ball across the room. The added drag of the o-ring seals on the powertube and front bolt were too much for the gun to overcome. This was the same case with 2 other guns (1 BT4 and 1 98C) that I tried it in to make sure. I had to return them because they were useless to me. However, I believe that with the spring kit (mentioned below), it might work properly.
The only problem I've had with this kit is that I had a hard time getting the velocity even up to 275fps... I ended up having to get a spring kit and moving to one step above stock strength to get velocity up to the best range. I believe it is something to do with the internal drag of the new kit. I have yet to sand/polish the sliding surfaces on the new receiver halves.
I use it with a 4oz CO2 tank and it has unrivaled mobility. Carbon Fiber barrel on the front and the small, forward CO2 balance it such that it's a 1-hander with no problem. The folding stock is pretty nice, but I'd have to say that I really don't end up using it. So far, it's what gets hit when I'm peeking around the right side of my bunkers.
Mags instead of a hopper has been a game changer for me. My exposure profile has dropped significantly and people can't get shots on me very well in a 1-1 shootout. I've won a number of 1-1's where the opponent can unload on me, but I get that one shot off and take the win. So far, I win more 1-1's than lose... It's incredible!
20rd mags is a limiting factor-for sure, but I like it. I carry 3 extra mags for a total of 80rds in a game. Before moving to this I was using a 50rd hopper to reduce profile. I don't shoot a whole lot, and I never have. I usually go through 17-30rds per game (4-6man teams). In order to make shots count, the only paint I use is RPS Evil or Diablo Allstar. Those two are the most consistent roundness (in my opinion) and have their best applications: Evil for open areas with little/no brush because it breaks up easily, but I've never had a bounce; Allstar in moderate or heavy brush because it pushes through to target, but fairly often, I have issues with bounces.