Could optimal tp size depend on the spring as well?
The mention of the 'Maximiser' reminded me of my own, new in about 1984, HW77k.
The Maximiser tuning kit was all the rage then and I bought one as all the adverts promised me it would transform the feel of the standard rifle. It was my first attempt at any form of tuning so I leapt at the chance. I sweated all morning to get it fitted. I remember the square section spring (Ox??) took a lot of preload and I did not have a compressor of any sort. But I did it ..... all fitted with the minimum of spilt blood.
What a transformation. From a pretty docile but bit twangy rifle I now had a rifle that bucked and jerked around in my grasp. I had no chrono so did not know what it was doing; but assumed it was still within spec. But I hated the firing cycle. Late afternoon I took it apart again and refitted the original spring. All became nice again ... the piston liner and 'debur' carried out as per Maximiser instructions had helped. No more twang and a good sweet firing cycle.
The rifle stayed like that for about 20 years when a chrono check showed power was declining so I put a new standard UK power spring in. A chrono check revealed power ott so spring reduction was needed. No problem.
I have never touched the transfer port.

Cheers, Phil