So first off I have to say that disk brakes are not that much better than rim brakes except in extreme cases. I would say DO NOT spend $250 for the upgrade, a %40 upgrade (of the cost of the bike) will not give you a bike that is %40 better, maybe %4.

The 8.2 looks to be right on par with the kaitai, save some very minor component changes and 700c over 650.

If you have money you want to spend, upgrade the components on you kaitai, starting w/ the shifters. Bicycles are more fun to ride when they shift properly. Having quality shifters and drive train components is the best way to insure that once a bike is setup, it will hold that tune and perform as designed.

I would also replace the suspension fork with a rigid one. that suntour is very heavy and is probably sapping energy from your stroke. If you are riding off road alot and feel a suspension fork is necessary, then put a good one on there.

If you dont ride w/clipless pedals yet I suggest you experiment with them while you have some cash to spend.

also all the little stuff you may need may be a better use of your cash instead of the disk brakes, ie seatbag, tubes, tools, patch kit, pump (frame and floor), lights, helmet, good lock.......