If i knew (or could find out) Matt i would tell you. I'm fairly sure the cams and heads were altered for the Mk2, i've heard rumours the Mk2 heads on the Mk1 engine make the best power of all but i don't know for sure. Also the engine management map was altered for the Mk2. I don't know how though.
I know Vauxhall did something similar with early 90s Corsa engines. The standard engine was the C14NE pushing 82bhp. They then produced at least 500 (for homologation purposes) C14SE engines. The publicly released figure was 100bhp but a source within Vauxhall confirmed to me they actually pumped out 125bhp - from a 1.4, 8 valve, multipoint injection engine! Basically they altered the cam, the engine management and a few other things - the valves were a bit trick to my certain knowledge and an oddball size. They had to be specially ordered from Vauxhall when the cheapskate owner skimped on a cambelt change and i ended up doing a top end rebuild on his Astra estate which is what Vauxhall fitted the unsold engines into. The others went for motorpsort and Police use.
Finding out the exact spec of the C27A2 is going to be hard now for obvious reasons but from what i've learned so far, the heads, cams etc are standard Mk2 and they use the same engine management. So it's all down to the lack of the cat and running on 98 RON petrol as far as i can tell.
A few tweaks you can do to your SLi - K&N E-0999 replacement air filter, available on Amazon ( http://www.amazon.co.uk/gp/product/B000E7DMEK
), water injection like i hinted at a few posts up, do 2 or 3 part - ATF changes using Carlube ATF-U then pull fuse "S" for 30 seconds and you'll be surprised at the difference! Also make sure all your breathers are clear, including the ones in the inlet manifold under the goldy coloured square panel on top, make sure the breather hose between the inlet and the front rocker cover isn't internally collapsed (replace using 10mm bore fuel/oil hose if necessary) and obviously make sure you've changed the engine oil and filter using a good semi synth 10W40 like TripleQX available from Eurocarprats. If you find your exhaust tappets tend to tap a bit at times, add about 500ml of Dexron-II D to the engine oil, it cleans and frees the hydraulic lifters so the valves open better, longer, wider aka more efficiently. I use it in all 3 of my cars (including the Jeep - made a very noisy tappet virtually silent) when necessary.
Most of this is really just basic maintenance but using the products i've listed gives the best performance and economy. This has been tried and tested, not just by me but others i know of and it works.