Horizontally opposed for aircraft

[Laguna]

Hey I was wondering if anyone knows what the highest horsepower horizontally opposed engine is. I am working on my engineering design project and need a set of engines for my aircraft. I am looking for about 1800 horse power out of two engines weighing 1850 lbs total. They can have a turbo but not a supercharger. I've looked elsewhere and can't find anything on any flat engines with this much power but I know that Subaru and Porsche run these type of engines pretty hard. I've determined I would need a flat 8 or flat 10/12. Because I can't find anything I was wondering what the draw backs of an opposed engine are that keep them from the 800-900 horsepower range.

Currently other air racers run a V or radial design and can get roughly 2.5 hp per lb of engine weight out of them with a turbo. Is that possible for a flat engine?

My engine knowledge is pretty much limited to inline fours... since I haven't been able to afford a better car