If I have this figured out correctly, but I may be off the deep end as well.

I'd go with the 1 in 9 unless you will only use the SBR for subsonic heavy projectiles. While the 1 in 7 does have a faster twist, lets look at this. A 1 in 7 twist means that for every 7 inches, the projectile makes one revolution whereas in a 1 in 9 twist, it is one revolution every 9 inches.

Your short barel rifle has a barrel length of 10.3 inches. This means as the projectile leaves the barrel, it has a twist rate of around 1 1/3 revolutions every 10 inches of travel compared with a twist rate of around 1 1/8 revolutions for the 1 in 9 twist for every 10 inches of travel. Lets say you add a 10 inch supressor to the barrel. This will give you a 2 2/3 revolutions for the 1 in 7 and 2 1/4 revolutions for the 1 in 9 as it exits the supressor. While stabilization in flight is a concern for the higher weighted projectiles at lower velocities, I do not think that it is sufficient to justify buying two different barrels - one for heavy subsonic or supersonic loads, and the other for lighter supersonic loads only.

I have a 1 in 9 in my SBR and have never had a baffle strike in my supressor. Of course I use a 9mm AAC pistol supressor on both the FiveseveN and PS90 SBR. Shot grouping does not appear to be affected by using the 9mm can on the FiveseveN or PS90SBR. No keyholeing was noted either - both firing a 55g Sierra Game King boat tail projectile at just under 1120fps. I am in FLorida and very near sea level therefore I can go to 1140fps before I reach supersonic speeds. You can calculate the speed of sound at sea level by using this conversion table:

http://www.srh.noaa.gov/epz/wxcalc/speedofsound.shtml
This is also a good source for calculations:

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe3.html#c1
However, as density of the air decreases or the temperature decreases(higher altitude, cooler temps), the speed of sound changes. At sea level, I calculated it to be about 27fps every drop of 25 degrees or roughly 1 fps for every 1 degree temperature drop.

The FiveseveN also has a 1 in 9 in a 4.8" barrel meaning that the projectile only twist slightly more than 1/2 revolution as it leaves the barrel or 1 1/2 twist as it leaves the 10 inch supressor - much lower than the SBR rates of spin, however, when it reaches the same distance as the SBR projectile, the twist rate is identical, it will spin one revolution every 9 inches verses the 1 revolution every 7 inches.

While the intention is to obtain the best spin rate to prevent yaw and thus keyholeing, for a 55g supersonic bullet, the ideal twist rate is around 1 in 14 for a .224 centerfire RIFLE cartridge if the information in this link is accurate.

http://www.nfa.ca/content/view/129/197/
As far as I know, the P90 comes with a 1 in 9 twist rate. It would seem to me that FN knew what they were doing when they built the P90 for use with both super and sub sonic projectiles.

You also need to consider that projectile velocities have just as much as an effect on projectile yaw as does the twist rate. So does the BC and numerous other factors. Thus it is extremely difficult to predict with certainty how one projectile at a sub sonic velocities will be different from another projectile at sub sonic velocities. The same holds true for supe sonic velocities.

Personally, I think that a 1 in 7 is too fast a twist rate for this platform unless you ONLY use subsonic loads.

My less than .02 cents.