The gun was plenty powerful enough. There wasn't much it couldn't punch through frontally, and in ambush, it could punch through the side of anything.

The important thing to remember is that although the M18 has tracks, a turret, and a gun, it isn't a tank. It's more of a self propelled anti-tank gun. U.S. army categorised it as a tank destroyer, but the difference is at the operational, not tactical level.

As a result, the TD crews were trained to do one thing above all else, that being destroy tanks. Other roles, such as bunker busting or artillery were secondary, and some units were authorized to deploy even without meeting the required standards for those secondary roles. As a result, it should not be a surprise that in tank-on-TD combat, the TD would score relatively well against the 'general purpose trained' tankers they were going up against. M10 and M36 crews enjoyed similar success rates. A slight corollary here is that by the time the M18s were in common service, the standards of German tank crews were deteriorating badly.

The other thing to remember is that, as anti-tank guns, they were purely defensive systems. The FM 18-5 of 1944 was adamant about this, even putting in italics for emphasis that TDs ambush tanks. Defenders always have an advantage, all other things being equal, and things were not equal between tanks and things designed specifically to kill them. So, fire a couple of rounds from ambush, in a site which M18 could get to quickly, disengage, and do it again down the road.

Of interest, the M18 was not universally well received by the using units due to the lack of armor. At least one unit flat refuse to trade in the old M10s. First and Fifth Armies generally preferred the M10, Third preferred the M18, this may be a reflection on the different types of fighting they encountered, Third army's war being more fluid.

It is also worth noting that very little mention is made of any tactical advantage gained by M18's high speed. It could get from A to B very quickly, but for obvious reasons crews didn't go charging around the battlefield at B at high warp factors. (Yeide states no mention at all, but I've found one or two comments in passing in the archives)