Luke,
That is why the A6 2.7TT is a good pick. You can have something mid-size (E-class, 5-series, etc) that still has the upgrade potential of the S4. BTW, according to the specs I've read on them (from Audi USA) there is only a 100-150lb weight difference between the S4 and A6 2.7. (automatic to automatic/manual to manual of course)

BTW, the upgrade kits for the S4/A6 2.7 are as follows:

stage 1: 295HP, 12.5 lbs of boost, $625
stage 2: 320HP, 14.5 lbs of boost, $1845
stage 3: 350HP, 17.0 lbs of boost, $4385
stage 4: 400+HP, 20.0 lbs of boost, $6895

All but stage 4 are street legal and seem like they wouldn't pose any significant problems for daily driver-ness. Besides, if you were to get something adjustable so that you can turn it down when she drives it...

Just an idea...Lee