We also explored the use of TruClient (both FireFox and IE) for performance testing purposes, but found similar result that you have experienced. The CPU and Memory footprint required per VUser made the protocols unusable for any real-world performance testing.
Exact quote excerpt from my written note to HP on the subject:
"...Additionally, we observed that our load generator servers were requiring to run vusers as dedicated processes (versus threads), and that each mdrv process would consume 300-500Mb of memory. With that sort of demand on the servers, it would be impractical to pursue the use of TruClient for achieving any significant load..."