How much would a system equivalent to your target workstations cost? Maybe it's a justifiable expense to get hold of a workstation that is more closely similar to your targets. ...perhaps eBay? ;)

I would be concerned that anything you use to load up the CPU of your current development system won't closely enough replicate the feel of working on a less advanced system. It's pretty difficult to consume exactly enough CPU cycles through brute force to mimic the feel of an older-generation CPU.