I had a modified 335 before my M3. My 335 had dinan suspension, wavetrac lsd and tune and several other mods. The lsd was great, loved the dinan suspension, but after a year with the M3, with no mods, i have zero regrets. Where the m3 really shines is on the t track, with the engine screaming and the beautifully balanced (and more stable) chassis, the m3 it's phenomenal. I'm not sure it would be much faster than the 335is modded, but it's more rewarding, more solid, has better seats, better rubber, better steering, better resale than modded 335, i could go on and on.
Worth the money? Depends on your financial situation, but if I could have afforded the M3 when i bought the 335, i would have bought it and I'd have more money and would have been much happier today. Hind site I'd 20/20 I guess.
BTW, my M3, ZCP, 6MT, leather, was only 59k, brand new. Not sure if you saved much money getting the 335is plus mods.