Hi all !

I bought a focus rs mk1 about 2 months back. It had supposedly had an engine rebuild a couple years before, the car has the full sabre 400rs package to my knowledge.
But there is a bit of history behind that, the person I bought the car off, said when he bought it, it was running awfully, he contacted the previous owner, and found out that the previous owner to him had been running hybrid turbo, bigger injectors, map etc….. but then stripped the mods off when he traded the car in. Now the person I bought the car off, contacted sabre and bought all of the mods the car should have had to match the map on the car, but bought a 400rs turbo instead of the 400r. ( All of this will make sense shortly ) Then there was a Dyno sheet for 380hp but the map hadn’t been adjusted
Anyway, 2 weeks into my ownership, I had a play down the motorway. 3rd into 4th. When I dropped it into 5th to slow down, the motorway behind me was covered in smoke, got it back to a friends garage. The tips of my plugs were melted away on cylinder 2&3. On a compression test, cylinder 2 was reading about 70 and barely holding, cylinder 3 was bouncing from 0-40 but not holding. Then 1&4 shot up to 140. Got the car recovered home, stripped it down and found it melted pistons 2&3. Chipped a Valve, melted alley over another 4, and the head needs skimming due to melted alley.
My instinct was a fuelling issue, I got the injectors tested and they were all fine. I don’t know much about the engines so I was hoping to ask someone more knowledgable for some advice of what could be at fault.
Sorry for the long post,
Thanks in Advance,
Sam