This is basically BrickMyth #1, that if you take into account the "magic shots" that
Jerry Reynolds (and nobody else I have ever seen) fails to count, that Kevin suddenly becomes pedestrian in efficiency.
It's not true. There's a very good statistic that takes into account those "magic shots" called TS% (True Shooting Percentage; it's a bad name, because it's not a percentage in any way). The simplest interpretation is that if you take TS% and multiply by two, you get the number of points a player scores every time he
attempts to shoot the basketball, fouled or not.
According to TS%, Martin is currently 14th-best all time in efficiency, scoring 1.21 points per shot attempt. Steve Nash (also at 1.21) is the only guy better among active players (minimum 700 points scored, career). 14th-best all time and 2nd best active is obviously very good efficiency.
Well, let's look at the numbers:
Kevin this season: 1.15 points per shot
attempt (ppsa, this would be #57 all time just above Ray Allen)
Kevin, career: 1.21 ppsa
Beno this season: 1.26 ppsa (this would be #2 all time)
Beno, career: 1.07 ppsa
Omri: 1.19 ppsa (this would be good for #22 all time)
Greene, this season: 1.15 ppsa (this would also be #57 all time)
Greene, career: 0.95 ppsa
Sergio, this season: 1.14 ppsa (#69 all time)
Sergio, career: 0.97 ppsa
So in fact these guys have been ridiculously efficient this year, Beno particularly so. Omri would be even higher if he could hit a free throw (if he shot 75% from the line, he'd be at 1.27 ppsa, good for best all-time

). But that's all this year, only a 22-game sample (5 for Kevin). Kevin is the only one who actually has a history of shooting this efficiently. If these other guys can keep up the shooting, then they do make Kevin somewhat redundant - but that's a big if.
I agree, hopefully Kevin can pick up the defense. Even so, taking history into account there's not a lot of reason to believe he isn't still our most efficient scorer.