I'm not 100% sure. This was one of my old WWAT whitepapers that was smartened up recently. The original whitepaper dates back to about 1998 or so. I'll have to find the original.
Back then, Client Access was a horrid overhead on an AS/400 system. These days, Client Access has improved somewhat.
The easiest way to test, however, is just to run a report on the AS/400 and watch the time - then run the report on a WORKSTATION and watch the time. The workstation communicates in the same way as an intel server - so if you see a performance improvement, you'd see a performance improvement on an intel server too.
Before you make the decision to go with intel application servers - try out this situation. There are reasons to go with Intel application servers if you're implementing JAS for callobject - or for XPi or some other interoperability - but JUST for UBE's its not usually necessary nor recommended.