135 brakes are cosmetically great and the only BMW brake that's a monoblock design.
The rotors are of a slightly smaller diameter than the 335s but you do get 6 pistons.
Probably not a worthwhile upgrade for a 335, definitely a worthwhile upgrade for everything else.
The front rotors are plug and play with most 18" wheels and above.
To fit the rear caliper and everything, you're looking at ~$500 labour and a new handbrake shoe (~$100 iirc)