Assuming the cylinder was on TDC and the valves are sealing OK, that amount of air blasting out of the oil cap can only mean one thing - a knackered piston. Likely a cracked ring land. Very common on remapped engines. Running that much boost pressure on stock pistons is a ticking time bomb.
Not a GTI piston, but an example of what it looks like. A hot compression test doesn't always tell the full story as the heat expansion can close the gap and the top compression ring is still doing it's job, but combustion pressure when the engine is running will blast through that gap very easily, and oil will also find it's way into the combustion chamber equally easily on over-run and idle.
This is why I've stuck to Stage 1. Even that is on the ragged edge for lifespan and operating outside of the original design parameters. Unfortunately with these engines, it's a case of when, not if, when running Stage 2 and higher.
