Well its either the car or the driver

As this was your first time and you're inexperienced with drag racing and don't really know how it all works (1. You thought reaction time counted in your times, 1sec reaction time off your 11.8 = 10.8. No, just no

2. You think you beat the RS6 because you crossed over the finish line first but dont understand how he beat you on the clock, 3. You don't understand the relationship between 60ft/ET, Terminal speeds and bhp )
That's not a dig at you Billy but just shows that you're a newbie to the game. For example, many that tune their cars go off their exit speeds in a 1/4 mile to determine their bhp over what a dyno will tell them. As you know, results vary from dyno to dyno, those that are into car tuning and drag racing will use the vbox (Performance box) and visit the drag strip and analyse drag strip times to determine what a car is doing power wise.
As I said in a previous post, drag racing is not about racing the guy beside you, it's about racing the clock. As a newbie, of course, the initial novelty is about racing the guy beside you but as you get more experienced you'll then gather its about racing the clock, reaction time then becoming irrelevant.
Crail on Sunday was running okay, believe me, getting a good dry sunny day at Crail is difficult to come by over the year.
Stage 1 375bhp Golf R = 11.8 (Bang on the money)
Standard RS6 = 11.4 (Have seen them do 11.2-11.3 so slightly slower than normal)
Liam Bryce's 550bhp S3 = 10.9 (Have seen him do 10.7 so slightly slower than normal)
Huracan = 10.8 (Expected time thereabouts, possibly 10.6 on a good day)
Stage 2 RS7 = 10.7 (Seen that car do a 10.6 previously)
Your RS3 = 11.6 (Way off expected pace)
As the day was so busy, tyres were getting cold and heatsoak from waiting in the queues probably affected times a little so no doubts on a quieter day, a tenth might come off those times.
No doubts, you're probably not listening to me now as I appear to have burst your bubble a little bit but 11.6 is not a good time Billy, no matter what way you look at it. A 375bhp Golf R recorded an 11.8 (which was around your average for the day) and he's about 135bhp down on you.
I recorded an 11.3 @ 122mph with 425bhp 6yrs ago. If my car had 510bhp, I suspect I would have been very close to the 10's in that car.
MRC haven't bumped you but when you run with passengers and record 1.9 60fts then that's your fault, not theirs. The car and tune are only part of the time, the driver and experience is another part.
I was looking very closely at your times on Sunday thinking low 11's high 10's but was way off the mark with that estimation. To me anyway, your car when you remove the passenger and launch well, may get you down to 11.2-11.3 but can't see it getting anywhere near the 10's. Believe me when I say, tenths of a second are very hard to come off, you need to decrease tenths but increase mph over the same distance.
Just for reference, here is a video of my old car on a Stage 1 tune (400) racing a friends TTS on a Stage 1 tune (320) On this particular run...
TTRS 11.7 @ 120mph
TTS 13.2 @ 110mph
1.5secs might not seem a lot when you view the figures like 13.2 vs 11.7 but look at the difference over the course of just 402 metres, I put about 3 artic lorry lengths on him.
There's plenty you can do with tyre pressures, tyre temps, cooling times, launch control release times, rollout, weight, fuel etc.
Some advice Billy, listen to the constructive criticism, I'm actually trying to help you here, the more you know about how to do it properly, the quicker your car will be and the better the time slips will be
I've already mentioned troubleshooting the Eventuri vs your standard airbox. I think Doug also mentioned this? Try get yourself a vbox (Performance box) and do some of your own testing.
Good luck and I hope to see you crack the times your car should be capable of soon