MPG I'm getting mid 20's in town and mid 30's on a run. It might not be as good as the 330d mpg wise but for the performance on offer I think it is really quite frugal.
Performance is not "way below" a 330i, they are the same speed to 60 and then the 335i is quicker above that and in gear times are also quicker due to the extra torque from the turbo engine. Plus you can remap a 335i for £500 which will give it around 370bhp which you can't do in the 330i. The only advantage the 330i has is in the throttle response which is sharper as it's an N\A.
However if you aren't a fan of creaks and groans I wouldn't get an E93, roof down they are fine but with the roof up, I haven't heard a completely silent E93 yet!