The Low Pressure Kit for the Tippmann (both A-5 and 98c have one) changes out some internals of your gun so you can run at a lower pressure (about 350 psi).  I've never heard of a High Pressure Kit, but stock 98c and A-5s run at the so-called high pressure (about 850 psi).

Actually the Low pressure kit has a lot of advantages with a Tippmann running co2 (obviously, HPA some has advantages over co2 - I'm not touching this...).  If you know you're going to stick with co2, then it's a good idea to get a LP kit.  Also, if you understand the phase chart of co2 and talk to the experts, you learn that co2 is much more stable when you shoot at lower pressures.  I have a 98C w/ e-bolt, LP kit, palmer stabilizer and anti-siphon co2 tanks.  This setup works very well.  I have never seen shoot-down when I shoot fast, and my palmer never freezes (the tank freezes pretty good when I shoot fast, though...)

By the way, many people say that an expansion chamber is a waste of gas.  It's better to get a good "co2 friendly" regulator (like a palmer stabilizer).  Obviously an expansion chamber is better than nothing, but there are better options (anti-siphon tubes are much better IMO, because they are much cheaper than expansion chambers, and they don't waste gas - the expansion happens in your tank, not in an external chamber).