THe 335 sport and m3 are equal in terms of regular moderate speed US highway driving or atleast I don't notice a difference. If there is cornering involved, sweeping ramps or obviously winding roads than its a different story.
M3 with DCT in 4th is "ok" if you are at 65-80 or so but for equivelent surge of power you need to drop to 3rd especially if your at 65-70 as that will be in the powerband of 3rd whereas 4th will be way before its powerband at 5k RPMS.
Honestly I went from a 335 and no matter how much I ready before its simply not natural coming from a low end torque turbo car to a high revving NA car to have to shift down so many gears. It almost feel like its bad for the engine or tranny or car in general as you are downshifting to quite low gears at pretty high speeds but frankly that is how this car needs to be driven if you want to get the same punch out of it.
Now I can only speak to a catless/tuned setup but drop it in 3rd at 65 miles an hour with DCT and you pull way harder than anything a stock or moderately tuned 335 can put out.
Personally I use 4th on the freeway for regular passing of moderate traffic but 3rd if I want max acceleration for passing or fun in general.
Keep in mind the higher you keep the revs, the more you get to enjoy the sound. It seems like there is an expotential increase in sound pleasure as you climb the revs!