There'sa lot of info here but not a lot of helpful stuff for your goal.
Back in the 80s, Beeman related some tests done on the R1, I believe... they measured the peak temp at over 1800 deg F... One of the major problems you run into trying to generate a lot of power is just that... piston seals burn quickly. Never mind all the forces at play here trying to compress enough air instantly to do the work for you. John Whiscombe went to automotive type steel piston rings in his later, higher powered versions for that reason but there are tradeoffs... as with everything. To move a slug down the barrel requires quite a bit of sustained pressure... most springers don't sustain that much pressure, even if they can peak pretty high.
On messing with the transfer port, be very careful here as that's what keeps the piston and chamber from destruction. Jim Maccari drilled and tapped the transfer port and used grub screws that were drilled to different diameters to fine tune that when he was doing power development. He's not a bad source of info if he'll talk to you... he tested a BUNCH way back to see what power was available but this was before slugs had become widely available.
I personally align with the general thinking here that it's not a rewarding path to go down... it takes a lot of pressure in a pcp to shoot slugs and springers just can't give that. Even a light slug is still difficult to push down the barrel compared to pellets... which are designed for more limited pressures. Then you also want to look at the ballistics... at what point does the slug actually give better bc's than the pellet? For example, A Daystate RW 22 shoots 20 gn slugs pretty well and also 25.4 gn pellets. In my experience, those slugs get blown more in the wind than the pellets do at longer ranges... so WHY would I want to go there?
Anyway, people pursuing the edge of the envelope is what got us to where we are today in most everything... if you choose to go for it... best of luck...
Bob