it'd be worth it if you have the money up front.
the z4 3.0si would still be a bundle of fun. One of my buddies has a Z4M and thats an entirely different beast all together.
my 330ci is fast enough that I didn't feel the need to modify the motor at all for a year.......the first car that's ever happened to me yet. put that same motor in a car, whack off 110lbs and make it more tail happy and it'd still be lots of fun.
that said if I had the capital this year I would've purchased an M3 instead.
and then on the other foot is the part where Timmy's car is one of the most fun auto-x cars I've ever driven and possibly the 2nd lowest HP car I've ever driven on the track so theres always that argument when it comes to if the base z4 is sufficient.
'03 VW Jetta TDI
'02 Chevy Corvette Z06
'99 Triumph Daytona