AFAIK, the main diff between the i7 and i5 is hyperthreading enabled on the i7, but I'm not sure if the software you're running would actually take advantage of the extra virtual cores. You already have 4 physical cores so the drop in clock speed might be noticeable. It will be fairly close, but all the benchmarks I've seen suggest that a i5 at 2.8 would be a bit faster than an i7 at 2.3 -- but it's hard to speak in absolute speed terms because it depends on what the application is doing.

It's typical to buy desktops for employees with high-demand apps like this cuz of the bang for your buck, but if your company loves spending money then pony up and get a thinkpad or something