I'm interested to know what you decide to do. I'm thinking of a tuning box because of its simplicity in install and removal. If Mercedes can give warranty on the Brabus box then I guess not all boxes are bad.
Racechips do one specially for the E400 M276 V6 and claim 415 PS. At £494 seems like a bit of a bargain compared with the Brabus box. I read too that they give a £4k warranty on engine and drive-train parts. Racechips are German made and TUV approved.
Anyone here using Racechips and what's your opinion?
With the exception being Brabus as it is integrated into the canbus system, with any other tuning box the ecu will eventually learn that it is being fooled!
With Kleenann tuning box it appears as being basic with old school manual adjustable settings (trial and error imo) hence I would opt for a customised remap any day from a highly reptuable tuner done live on a rolling road.