The thing is, you definitely are delaying your Full Thrusts by at least 0.2s. Because macroed abilities cannot be queued, you must press the button after the GCD is complete. Whether you perceive this or not, this will result in a loss of at least 0.2s per macroed GCD action.
Click image for larger version. Image shows timelines of a 5 GCD microrotation (one 1-2-3-4-5 combo on DRG) with and without macros. Macro timeline shows GCD drift due to action locks.
When not using macros, you can press an action button up to 0.5s before the current action finishes. This enqueues the action for immediate execution as soon as the current action lock releases (this is 0.7s for most instant GCDs and oGCDs, but closer to 1.0s for Jump family actions).
When you use macros, your actions do not queue. If you press the macro button before the current action completes, you get an error. This means you must press the button after the action lock releases. If you are able to press a button 4 or 5 times per second, this will cause a delay of about 0.2s every time you want to execute an action.
This is not an issue for a single oGCD action since the GCD gives you about 1.0s more than you need for the one oGCD action; however, all macroed GCD actions are affected by this delay. Worse, if you have two oGCDs that should be used in the same GCD, the combined delays will delay the next GCD action even more than it would have been just on its own.
You might not perceive this or you might not think it's significant, but it is happening if you macro combat actions. Even if you didn't have additional drift from double weaving oGCDs, the delays from macroing GCD actions would add up significantly.
Maybe you're only using one or two macros right now, but to put it in perspective, let's look at what would happen if all your GCD actions were macroed... Each 5-action DRG microrotation would accumulate at least 1.0s of drift. In a 7 minute fight, with a 2.4s GCD, you can typically use 175 GCD actions, or 35 microrotations. If we add up all those macro delays, you would lose 35s, which is 15 actions, or 3 whole microrotations. That's a lot--about 8.6% of the DPS you could put out if you just didn't macro your combat actions. This is a loss of something like 1.3kdps at current endgame.
tl;dr: macro GCD drift is real whether you believe in it or not, and macroing your GCDs will result in a loss of at least 1kdps at Lv80.