I know what you're saying. I know the M3 is a great car. But even by your own admission you say you need to stay above 5.5k to see the major benefits. I drove loads (6+) during my 6 month search. If I had found one I wanted for the right price I would be sitting in one now.
I'm just glad I drove the 335i and was able to look at it from both sides. I just couldn't justify the huge price premium in the purchase cost and the fuel/service/tax. Not for the few minutes per drive when the M would be the better car.
I have done over 10k in the 335i since April. Sat at 70 to go and see Makkan all the way down the A1, I got 40+MPG. Last week on a trip to Germany to get my son I hit 172 on the Autobahn, before my better half bottled it. Many a mix-up with "more obvious" "fast" cars, all have been surprised when I leave them behind.
After a chance encounter with a lovely E92 M3 who happened to be a friend of my detailer. He contacted me a couple of days later saying he was looking at changing the gearing, as he was amazed how well the 335i pulled away from him at all speeds.
I guess it's all about what you will use the car for. If I was using it for weekend blasts into the Lake District and tearing up B roads them I would prefer the M. But in everyday driving the 335i is the better option in my opinion.
If you want to make any performance mods, then the 335i is the only way to go.
WM
Edit:-
Google "thinking man's M3" Plenty of real world info out there.
WM