The slower a bullet is traveling the faster the twist needs to be to get the same number of RPM's for stabilization.
So if you are shooting subsonic ammo, you will need a faster twist for the same bullet length and weight.
Over stabilizing a bullet will only hurt the accuracy on a precision rifle, you won't notice it on a PS90, but under stabilizing as you know will cause keyholes and, if shooting suppressed, baffle or endcap strikes.
I've been lurking on and off for about a year and a half now. I spend most of my time on silencertalk.
I emailed them on this topic and here is what they said:
The original PS90 barrel is 1/9 twist. If you plan on
shooting factory loads, you want to stay with the 1/9.
If, however, you're planning to consistently shoot
subsonic rounds that you're reloaded, you'll want the 1/7.