VERY interesting. I will double check mine and get back to you on this. Sounding like the bigger sleeve may be in order.I can probably do for you what I been doing for the B-Chassis guys so you dont have to buy a new prothane mount and the probably just repeat itself and make you a larger sleeve here is a picture from Pat Klutch and a stock B chassis rear prothane mount...
What your looking at here is a new larger sleeve pressed into a prothance rear B chassis mount and also a side by side comparison of the larger sleeve vs the stock prothane sleeve there is a big difference, this will basically rejuvenate your oblong mount and put back the preload if you will back into it which it should have had much more of from the start. You cannot compare B-chassis vs P-chassis they both start off with different size sleeves and the P-chassis stock prothane sleeve is basically the size of the upgraded B-chassis so the P-chassis will be even larger, i'll send you a pm