Increasing the length of the extension increases the effective weight of the carrier (tongue weight). If your carrier plus bike weighs 425 lbs and your hitch is rated at 600 lbs. you are fine. Now double the length of the extension from 15 inches to 30 inches and your effective tongue weight doubles to 850 lbs., exceeding the rating by 250 lbs. But you are not doubling the extension, you are only adding 1.5 inches. Therefore 15 inches X 110%= 16.5 inches. so you are increasing your effective tongue weight by only 10%, or to 467 lbs. instead of 425, still well under your rating of 600 lbs. No worries, PROVIDED YOUR MAX TONGUE WEIGHT RATING IS 600 LBS. or better, which you did not say. You need to measure the distance from where a standard hitch ball would be to the center of where the tray was before the extension, to account for the increase due to using the second hole. Let's say that was 18 inches. The 1.5 inch increase equals 8.5% of the actual weight of the carrier plus bike. You will have to weight the carrier and the bike separately, my calculation is only a rough guess.
If your rig worked fine with the carrier originally, no front end wandering or rear axle bottoming, the 1.5 inch extension will probably not be noticeable. If you were already experiencing those problems it might get dangerous.