As others have pointed out, I would put a lot of stock in the words of a bike shop employee who is trying to talk you OUT of giving him your money. I think his idea about demo-ing one with identical geometry is excellent. Then you won't have to wonder whether you'd be comfortable with the fit. You should try to make sure, though, that the demo is a thorough one -- not just a loop or two around the parking lot. When back and neck issues arise, they don't usually appear (at least for me) until after 30 or 40 miles.
Also as others have said (and as you well know), a new bike won't make you faster. If it is a sweet bike that you're really excited about, however, it will make you want to ride more. Plus, it is just fun -- look at how giddy DrPete is with his new Pinarello purchase.
I'd suggest following the advice of the guy at your LBS, and then making an informed decision when you have ridden the demo. Then if it turns out the Pinarello is right for you, then by all means
go for it!
