Wow. Well, yeah, sounds like you might benefit from a beefier cassette. Your riding will improve, however, if you take the mech's advice and pre-shift.
If you poke around, you'll see that the xtr's are pretty darn light, and a few of the sram's are a bit heavier. Neither are cheap. Not sure which cassette your mech is recommending, but it's pretty much impossible to stay on top of the shimano vs sram debate year in and year out. At least for a mere mortal such as myself. One year it's all about sram... the next it sucks donkey balls. I will say, unequivocally, that shimano's brakes the last year or two have been killing it.