I have had my car remapped by Upsolute and also tried Carlsson Filters and Brabus exhausts and filters. I also have sprintbooster fitted.
My conclusions are that chipping adds a little more driveability and perhaps a little more "smoothness" but thats all, talk of 20bhp, 30bhp even 40bhp is all rubbish. I have now had my remapped to the last w203 software issue and can't feel any difference.
Having tried 3 different airfilters.
K and N oils up your MAS causing it to fail. I had my car dynoed with a standard airfilter and a K and N and there was NO difference at all. The Carlsson one if just a french made K and N. I now run the OEM one but change it every year.
I have tried AMG and Brabus exhausts. Both added noise but NO power increase.
Sprintbooster. Is excellent it adds no BHP but makes the car feel much quicker through the gears.
I'll get my car dynoed some day with SB fitted and see how the torque line looks because it goes a lot better than it did without the SB.
The other M271 engine mods are headers (manifold) of which Kleeman makes a gorgeous looking one for £750. Unfortunately it discolours the first time you use it and therefore loses its looks and if you read the kleeman website it isn't liscensed for road use and is used at your risk. Other headers exist but I've not met anyone with one fitted.
Schrick (sp) make cams that give your engine more top end but less mid range and sometimes muck up your idle.
Several firms make pulley kits that the US forums seem to question the worth of. If they work then why are there always used sets available on ebay and US forums.
MKB do a pulley, intercooler, chip, filter kit for about £100,000.......
Which leaves Kleemans K Box which kids the kompressor it needs to be full on all the time and alledgedly adds 40bhp or so. I've never met anyone who fitted it (£750 ish) and US forums have people selling them...why?
I'd have your car well serviced, fitted with new and correct plugs not gold plated titanium turbo plugs, I'd get a new MB airfilter, fit Sprint booster and enjoy your nice reliable car. If that still isn't enough don't spend 000's on modding the engine in order to make it unsaleable just trade it for a 3.2 or an AMG.
HTH
An old experienced modder!