So I finally got a Stage 1 BMS tune for my F30 N20 328i this week (from an end-of-lease forum member). 91 octane and electronic waste gate, unfortunately.
Overall: Pretty happy
Gains: I am happy to report that the gains are significant, and can not only be felt by the butt dyno (esp. at medium speeds), but can be verified through increased illumination of the traction control light (lower speeds), and through the pocketdyno iPhone app (~35 whp by the end of 3rd gear).
335i vs 328i: I highly doubt it's now up to par with the 335i, but It's been a while since I drove the 335i.
Throttle curve: I feel no change at least up to 50%, but things escalate quickly afterwards. Kind of like the eco-pro throttle map in comfort mode. Even though it's less linear now, I like it because it means regular gas mileage for normal driving.
Lag: The engine lag MAY have been reduced a little bit, but I'm not sure - it may indeed just be a placebo effect. I also feel the boost MAY be overshooting (power goes up momentarily but someone clamps it). I wonder if the lack of wastegate control has anything to do with it.
Closing remarks: A friend of mine (with an n20 x1) liked it so much, that I lent him the box for the weekend (talk about promoting a product for free!). Also, if you are wondering, installing and uninstalling the tune is THAT easy! The air filter / AFM layout was slightly different on the x1 BTW...