The best thing you can do for a Boxster's reliability on track is stay off R-compound tires. You won't be as fast, but you'll still get the full track experience. As long as you do that and watch your temperatures, with the mods you had, you should be ok.
I wouldn't swap a 2.7, it's a lot of trouble for a marginal gain, just do a cost-benefit analysis on the 2.5's rebuild vs replacement vs selling the car.
The 2.5 is great on track due to the cable throttle's sharper response. The 2.7's E-gas is pretty quick for a DBW system, but not instant like a mechanical link. This is by design for emissions purposes, coded deep into the DME and can't be rectified with anything short of an aftermarket ECU.
Most of the 2.5's known issues were catastrophic failures that happened quickly and affected few engines. If your engine was going to D-chunk, it would've happened 20 years ago. If it made it to 2024, it's pretty safe to say it won't happen. The 2.5 also has a lower likelihood of bore-scoring due to stronger piston skirt liners, and the oiling and cooling systems are near-identical to the 2.7, with no drawbacks.
Install a 3.2 center radiator and you'll have the coolest-running M96 on track as long as your system isn't already broken via a leaky reservoir or loose impeller chunks floating around clogging it.
It'll be a lot less trouble to stick with the 2.5, you won't have to swap anything around and all the advantages you mentioned do apply, it does in fact have a stronger IMS and thicker cylinders. There are ups and downs between the 2.5 and 2.7, but it's not a worthwhile swap unless you REALLY want the extra 15-20 horsepower/torque and don't mind the E-gas.
For a rebuild, I agree with The Radium King that you can replace just what's actually failed and try your luck.
As for a replacement, I'd either replace parts while it's on a stand or just put it in the car and drive while you rebuild the old one on a stand as a backup. Doesn't hurt to have 2 blocks, you can fix one up while you run the other. At least that way you can go from 90/10 wrenching to 50/50. Not quite the 10/90 you wanted, but if you do it right it should become a 10/90 eventually.
If you're just sick of wrenching, though, I'd get a track-proven Miata, MR-2, 3-series or the like and run it with routine maintenance. It'll be slower than the Boxster, but there's no such thing as fast, cheap AND reliable, you'll have to pick two.
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