I agree - the performance increment from a stock 135i to 1M probably isn't worth the cost to upgrade. Certainly the benefit is even smaller if you have added a tune and maybe some suspension parts.
But if you don't already have a 135, it's an easy case to make that the 1M is worth the incremental price. The brakes, suspension, diff, body, slight tune ... no way you duplicate that for the price of 1M minus 135.
... a glorious V8 that screamed and hollered as the revs rose and then howled in an orgy of what sounded like BDSM ecstasy as it neared the red line.
Well, you can forget all that. The new car is fitted with a turbocharged straight six. Turbocharging? In an M car? Thatís like putting gravy on an ice cream. - Jeremy Clarkson, discussing the S65 and then S55 M3 engines.