I recently replaced the spring and breech seal in my Senior and it has increased the power but not dramatically. After reading the OP I measured the hole in the new rubber breech seal to be 3.5mm
With a 4.5mm barrel I would have thought this is a restriction but don't know enough about fluid dynamics - tempting to drill it out to 4.5mm but I'll do a bit of googling on transfer ports and breech seals!
I know what you mean about introducing plastic into a Senior - it doesn't seem right even using a rubber breech seal. I would be tempted to leave well alone and just get something else if you need more power. Any gains you get will be so minimal anyway. It is frustrating that the piston ring doesn't seal a bit better though - but I wonder if the stepped design seals much better under power - ie at slow speeds it isn't compressed against itself or the groove it rides in.