We have the same problem... we own 3 Cat systems, and so far we did to improve speed.

Hard Drive is the main issue...
So if you have 1 movie playing back from 1 Hard Drive is perfect
if you have more than 2 movies, on our experiences, if you can put a movie in diferent drivers like Movie 1 in HD 1 and movie 2 in HD 2 (even if a system resides there) you will se Cat playsback no problems...
But is crazy to spread out movies like this, so we did set up a RAID 0 system, in our particular case, for monies reason, we did a 4 Firewire 800 External Drives with a SoftRAID solution (Apple has his own in the system, but we use SOFTRAID 3). That way, we could reach 3 to 4 movies playingback togheter without loosing speed. Depends of the codec etc...
The best looking solution is a External SATA HDs, set up as a RAID, connected to teh system thru a SCSI Card, 4 8 or so.... but that is Expensive.
Another Solution is a RAID Computer, like XServe from Apple and Run thru Fiber optic Cable (Networked). I did never tested... but all sound good.
We are currently using V4, and maybe is a impression, but V4 looks like plays easier the 4 moviens than V3.3, but maybe i am wrong.
Remember, CATALYST is a NON DROP FRAME software, different than Quicktime (normal config). I belive if you set up your Quicktime to PLAY ALL FRAMES option, you will get results closer to the CAT sys. also depends on the Graphics used, Effects etc...
Is hard to Play movies on computers, ge3tting better, but not quiet ready for a price.

Remember, when you play a movie, release all the non show layers.

Hope that helps...
Gian