£6K? Eeep, not a lot out there that's much faster. Impreza sure, but you'd need a remapped STi and they're either going to be a dog or too pricey.
Is a 335i really that much quicker than a 350?
You might get an E46 M3 for £6K, but like Brillo correctly says the M tax is horrendous.
If you can stomach FWD, a tweaked Lean Cupra R is going to muller a 350 in regular driving.
Personally I'd forget speed and just get something different. Ever had a convertible?