That' s a good arrow weight but not one I would choose for all out speed . Since the LX is not out yet it' s hard to say but from all the Mathews I' ve shot the last 4-5 years , you can easily shoot 5.5 -6 grains per pound smoothly and quietly. I shot a 393 grain arrow with my MQ-1(72#) at 295 fps , I shot a 495 grn arrow 257 fps(MQ-1 70#) with it. I noticed no difference in smoothness or quietness . I' ve shot an MQ-32 at 4.5 grn per pound ,which was incredibly smooth. I recently have shot the 393 grn arrows with my Ultra 2 at 296 fps and a 350 grn arrow 308 with no noticiable noise difference and only a slight diiference in vibration.
Do I think you should change arrows .....no !! But if your not happy with the speed when you get your LX ,I think you could easily shoot a 375-400 grain arrow and pick up speed without any noticable difference in shot noise or smoothness. Just my $ .02 .
Good Luck ..........I can' t wait for my LX to get here !!