Einser M has the Wagner drop-in intercooler. It may be best to contact him regarding his thoughts; I believe his 1M is tuned.
In regard to performance, I cannot disgree with the other's in here, they are spot on. To gain the most, in performance terms, from your engine with a better IC fitted it is best to have a map tailored for it so that the turbocharger can be pushed a little further. That said I wouldn't say though that it wasn't ever worth while fitting a more efficient/large IC to a stock car, intake temps and creep even in mild Summer's and during consistent hard runs in any climate. Sure you may not get the power bump that an IC tailored tune will utilise but you'll retain your power for longer. It's never a bad idea to fit a 'better' IC and you'll find that most of the IC's made for the N54 engine aren't really large enough to provide any major lag.
BMW 135i N54 | Black Sapphire | M-Sport | BMS 10mm Spacers (Rear) | Eibach 5mm Spacers (Front) | Gunmetal 261M 18" Wheels | Vredestein Ultrac Sessanta 225/255 | LCI Tails | COBB AP Stage 1+ | Matt Black Grilles | BMW PE + Resonator Delete | N55 Midpipe | Rieger Tuning Matt Black Diffuser | ETS FMIC