I think maybe the SL55 fuel pump 7.00228.51.0 is the way to go....although NW's also did the Walbro with the Waldig Mani/FPR setup last time I looked.


There is a new 7.00228.51.0 in Australia for sale which you may find a bit cheaper due to the current exchange rate of only 1.42AUD to the USD....I think it works out at about USD$250...I was looking at it just yesterday...I'd change that and run it before you put in your 550's....
Pierburg Fuel Performance Pump 360L H Better Than 044 | eBay
PS - I've never been able to find a pump curve for the stock pump...but I have one for the 7.00228.51.0


also re the lean issues...see this.....current thread over at the SLK230 BBS...which reportedly uses the same fuel pump as the SLK32/SRT6... - Subby's up in the 12+psi range though v stock which is 6.5-7...
air fuel ratios, thinking of going 5 bar FPR - Mercedes-Benz Forum
Inference from one of the posts within that is that its in the map rather than the delivery...but I cant see that still hanging around after a retune.