88mm L/56 FlaK36/KwK36
Muzzle Velocity: - 820 m/sec.
Armour Penetration: -
APC - 105mm@1000m@60deg.
APCR - 103mm@1000m@60deg.
HEAT - 165mm@all ranges@60deg.
90mm M41 gun (as mounted in the M48 medium tank), L/50
Muzzle Velocity: - 884m/sec (APCBC) M82, 853m/sec (APCBC) M318, 853m/sec (HEAT) M348a1, HVAP (APDS) 895m/sec.
Armour Penetration: -
HEAT - 190mm@all ranges@60deg.
M332A1 APDS = 160mm@unknown range@unknown (more than likely 500m@60deg).
I'd also recommend consulting -
http://www.wwiivehicles.com/usa/guns/90-mm.asp for penetration values for earlier US 90mm guns.
Not really comparable on the basis of the sketchy information available that I've found on the 90mm M41.