Well, they are about three inches longer...more coils
Sounds like they might contain a bit more than 5% more energy.
Why did you opt for the heavier springs, were you experiencing feed problems?
But the 6 round mag and 7 round magazine of the PM9 is a different spring. they are not identical..
It's a matter of case diameter, not capacity. The T9 mag holds 8 rounds, but because the .40 is a larger round the mag only holds 7. The spring compresses about the same distance regardless.
Judging by today's left wing, looks like Senator Joe McCarthy was right after all.