TGT said the cover bonus gave him a +5 to AC.  If the improved Cover Bonus gave say, a plus 10 to AC, some house rules would say Kasumi missed, since I've played house rules where a Nat 20 isn't an auto hit, but adds 10 to the attack roll.  If TGT is using that house rule, then the next time Kasumi rolls a nat 20, she should add an action point to it... maybe that would then be enough.