I've always (well, since Pentium days at least) said you don't actually want your devs using top-end machines, at least for testing and debugging. You want them using the minimum target hardware. That way they'll actually care when the product slow on low end hardware, which will make it usable there and snappy on high end hardware. I've certainly caught a few accidentally-exponential style UI issues which were unnoticeable on a new workstation but very obvious on a slower machine.