These are my thoughts...
I think the 350z DOES cut it with most of the other cars especially the Supra, S3, RX7, S2000 and to a certain extent R34 and Cayman S. I'm sure Liam will tell you that after following the 500bhp Skyline through bala.
The selection above are tottally different cars (hatch, coupe, 4 door saloon etc..) and it would be really up to him to test drive them and see which is best suited to him. I don't think its all about speed which is the impression i get from the above, i'd want something that excites you everytime you get in which i don't think you'll get from a S3 maybe if it was modded alittle. Anyway,,,
Cayman S - Quality car, will hold value very well, a few modded ones about that are as fast as a 911 turbo. Slightly expensive if your modding?
R34 - Big bills if anything goes wrong but fast standard, and can be made to go like S**t off a shovel.
S2000 - Need a turbo or Supercharger to get any real power gains, in similar boat to 350z really. Will hold value quite well, and voted the best used car 2 years in a row.
M12 Noble - I'd only get this if i was throwing money away. Great car but huge bills from what i've read. Not something i'd like to drive in daily, but super quick.
RS4 - awesome car, will do everything you need of it and won't break being German. Residuals very good. Do you need much faster? Great a-b car. Naturally aspirated so will need turbo/supercharger if you want more power. Single figure mpg when pushing.
S3 - Would be a sleeper similar to the Saab. Very easy to mod. Just check out http://www.amdtechnik.com/ I'm sure they'll have some good stuff for it soon. Expensive to buy though compared to others on this list. Will hold value quite well. Boreing to drive?
RX7 - Still looks good today as it did on its debut. Running costs will be high. Can be tuned to over 600bhp though. Abit old me thinks.
Supra - Again Looks great, especially some modded ones. Big power again. Showing signs of age?
350z - Can be had for 15k now, bang a turbo on all for less money than the S3 and your flying. Looks stunning standard, and looks even better modded. Running Costs will be reasonable compared to some others on the list too.
My top three in no particular order would be:
350z
Cayman S
RS4
Anyway, modding will very rarely gain you money and quite often lose you money. So whichever car he mods, i doubt he will get it back on the resale.