Nobody ever reported this, and it's very well known that GSX doesn't cause any visible impact on frame rate.
Don't know anything about that Accu Feel product you are referring to but, I guess that if there were problems between it and GSX, we should have heard about it by now.
As a general rule, GSX being a native FSX products uses Simconnect and, if other addons are using it too (in the wrong way) by clobbering the command pipe with too many commands, it's possible that GSX can't get enough time share to send its own commands, because other addons have monopolized the Simconnect communication command. We saw such issue with an earlier version of KDFW when used together with UT2 and, due to the large number of Simconnect commands sent by UT2 in a short period of time, resulted in scenery stuttering and delayed updates.
Maybe this is a similar problem, the most obvious test to do, would be temporarily disabling that addon and see if it makes any difference.
