Unfortunately I don't think there are any options for shortening the exhaust header. (technically a "manifold" is multiple headers combined into a single part, derived from the word "many-fold", so the word only makes sense when talking about multi-cylinder engines.) The stock header is a simple 90-degree downward bend and a 90-degree forward bend; there isn't any extra length to get rid of.

Good to know about the compression ratio. That's something I can continue to experiment with, then. Would you recommend removing the shim on top of the sleeve, or the shim under the lip of the sleeve, as the next step? I honestly can't decide whether the shim under the lip of the sleeve is actually increasing the exhaust and transfer port timings enough to matter.

Though I will say the shims definitely serve as gaskets as well; I had a different engine that was blowing bubbles under the cylinder head, and it didn't stop until I sanded-down the top of the sleeve a bit to ensure it was flat and then tightened the hell out of the head bolts. I actually had to tighten the head bolts while the engine was running so I could see where the bubbles were coming out, and tighten the bolts accordingly. The good news is it never leaked again after I did that, even after taking the engine apart a couple times.