Oh no!!!!
Benjy...just do TWO things for me (well, it's for you really) please.
1. Make sure your map comes with a money back guarantee.
2. Have a dyno independently done before and after at a place
different to where you're having the map done . This last bit in bold is most important. For this around the London area I'd probably recommend Evolve in Luton or Surrey Rolling Road.
Only then will you be sure you've got value for money as you'll always 'feel' it's quicker after a map....it's the placebo effect! Indeed, I was sure my Brabus tuned car was faster at first but the telling point was when I had that taken off and the standard map put back on....the car didn't feel one jot slower!
Tuners often quote big gains for the C63 as its based on a standard 457bhp one - the PPP basically already has the state of tune that you get with a remap. If they have lots of experience with C63's you can bet your bottom dollar it won't be on PPP versions.
You know all about my Brabus upgrades and the fact is that all tuners do a very similar thing with the ECU....theres nothing much someone else can do that Brabus won't have done to get power safely from that engine. Yes, it likely can be possible to get more power but you need to be careful how some may do it - some tuners make the car run very lean to make more power yet it's not very safe for the engine. Hopefully one of the other guys who gave me a lot of technical feedback will pop along to chip in with more info on this.
With regards to the E63, the main reason it has more power is due to the intake and exhaust system - the C63 suffers from a cramped engine bay and as such creates a lot of heat and it just doesn't breathe as effectively. It's the same with the SLS - that car has a very fancy induction system which takes up a lot of the space at the front of the car and this again is where a lot of the extra power comes from.
Sorry to put a downer on this and I hope whatever you do you're happy with it (well, even I saw some bhp increase - it was only 4bhp but it was up at least). I do know the feeling of having the draw of the promise of more power by remapping so I know exactly how you're feeling and nothing I say will likely change your mind

(if it was me, I'd be stubbornly going ahead with it anyway

).
However, after having umpteen cars remapped (nearly every car I've owned and I've never, ever seen more than a 10bhp increase on any N/A car) from a variety of different tuners, I am extremely sceptical of any claims on a naturally aspirated car....I sort of feel that I've been through the pain so other PPP owners don't have to!