There are a few different reputable and well known tuners as far as the S550’s go. Top of the heap as far as recommendations and hype go are probably Lund and Palm Beach Dyno, they’re both super reputable, and for good reason. Give @beefcake a shout when you’re ready for a tune. Super reputable vendor in the community, wealth of information, great prices, you can’t go wrong.
Agreed, by far tuning for e85 is the best single mod bang for your buck, but pairing it with additional mods like long tubes only enhances the performance.
I would highly recommend Palm Beach and Lund. You can not go wrong with either. They have both tuned my personal cars and I have no regrets period. Made 750whp on a Procharged 2011 GT and 1200whp on a 2017 with PBD. Hit me up if you would like to discuss some some options. Lethalperformance.com