how bad is the latency issue with digi 002 rack ?

I don't hear any noticeable latency when using a small buffer size. I set mine at 64 and forget it. Once you are done recording increase the buffer size to get more plugs and stuff.

the latency on a PT system does not increase with/is not dependent on track count. It is determined by hardware buffer size, which can be lowered to the point where latency is not noticeable.

The Mbox nor the 002R have noticeable latency. It is a function of the speed of the computer which would let you use small buffer sizes. If your computer will let you use 64 or 128 sample latencies, the 002R and the Mbox will have this low latency. I use both and play while monitoring with no audible latency.