Bit of a summary about what happened at Oulton then..
I had the exhaust made on Friday - I wanted it 3inch all the way back and not boomy/loud. Ended up putting a 100cell sports cat (that I already had) up front with a backbox, then just a 3" straight tip. Whole thing was TIG'd, really nice job, happy with the sound too. You can see it in the previous posts.
Arrived at Oulton on the Saturday, all was well. Took it out on the sighting laps and it certainly spins the rear wheels up very easily when the boost comes in (only stock at the moment!). Anyway, after a few laps I started giving it a bit more throttle (plus there was a dry line by this point), but it just didnt want to boost past 4000rpm, it was like the engine didn't want to rev, or the turbo was dropping all the boost.
Everything else felt OK, suspension is really nice, turn in is good despite not even being set up fully yet, brakes are good too. After a lot of messing around with boost pipes, checking if the actuator was OK, running it without a boost controller, running it without a recirc valve, checking all the vacuum hoses, it still didnt want to rev up when put under load. I originally thought it could be fuel (before checking any of that stuff), but ended up thinking it wasn't after testing it at the rail and it seemed to have good pressure. You could rev it up to 7k when stationary, just would not pass around 4000rpm once moving, and it had very little power.
After testing all the above and it making no difference, I decided to remove the pump from the tank and try it in a bucket of fuel. When it was inside the tank it was very loud (high pitched) but I thought that was just how it was (Walbro 255 from an S14a). After running it in the bucket it was silent, which kind of pointed towards this being the problem. I removed the filter from the bottom of the pump and it was full of sh*t - I think it had been sat on the bottom of the tank and the pump was just sucking against the bottom, not getting enough fuel - hence not being able to rev up under load. You could put as much throttle as you wanted on but the revs would not change.
By this point it was nearly 4pm, so I decided to re-fit it into the tank without its filter as a test. Started it up and the pump was near enough silent, engine ran smoother too. Turns out that damn fuel pump was more than likely the cause of the whole days issues. Oh well!