The N57 3.0 straight-six diesel in the BMW 335d is one of the most under-rated tuning platforms BMW ever built. Stage 1 takes it from 286bhp to ~360bhp safely.
Real numbers
Stock: 286bhp / 580Nm. Stage 1: 360bhp / 720Nm. 0–60 drops from 5.6s to ~4.9s on a 4WD xDrive.
Why it's safe
The N57 turbo, injectors and ZF8 gearbox all have headroom built in for the 335d-as-535d (same hardware, different software). You're using factory margin.
Gearbox concern
ZF8HP handles the torque, but a TCU tune is recommended on hard-driven xDrive cars to prevent clutch slip under launch.
Daily drivability
Better part-throttle, stronger overtake, and (driven gently) no MPG loss. One of the best ROI tunes on the BMW range.
Related services on StageMyCar: Stage 1 remap and BMW remapping in Harrow.