Unfortunately, there's no way to do that. StrokeIt maintains its own priority level to ensure that it will run smoothly while still using the fewest resources possible. Under idle priority, gestures become slow and lagged.
Have you had any problems with StrokeIt using too much CPU or other resources?
There is a known bug in the .9.2 series that sometimes causes StrokeIt to run at High priority. This has been fixed in the upcoming release.
-- Jeff