Haven't you seen the microSD to usb converters?
I actually got one with my 8gb card I purchased about a year ago for $20. Pretty handy, turns any mSD card into a USB flash drive basically. Installed Win7 using this method on a makes computer the other day because he doesn't have an optical drive.
With that said, I would imagine that using the TP2 as a mass storage device would slow it down over using that converter I mention above. I know my TP2 gets pretty freakin hot when I do a big file transfer. The drop in speed probably won't be significant, I'd just worry about the phone itself
.