OMG dude. Ok, listen to this:
A B-17 is well armored right? Ok, a fighter against it in its day (1943) could be a FW 190A-5. Do you have any problems killing the heavily armored B-17 (more armored than a TBM) in a FW 190A-5? NO. You do not. Why? Because the 190A-5 is a superior aircraft. Armor counts some, yes, but not enough for that to be the sole reason why we keep the TBM-3 as a TBD-1. Even the well armored 1944 ride that is the TBM-3 is able to die to an A6M2. Granted it is not easy and eats up a TON of ammo, but still possible. My point is, it may be more armored than the Kate but not enough for a case against using the B5N.
Cool story.
You're making the case that certain performance aspects of the TBM make it an unfit substitute for the TBD (it's predecesor, fwiw). Your OP listed three main categories of performance discrepancies that support your claim.
TBD-1 TBM-3 B5N
Max Speed: 206mph 262mph 235mph
Climb Rate: 720ft/min 1,540ft/min 1,283ft/min
Range: 435mi 985mi 1,237mi
Armament: .030 nose, .030 tail 2x.050 wing, dorsal, ventral 7.7mm tail
Ordnance: 2,000lb torp 2,000lb torp 1,706lb torp
You're essentially saying that in judging a torpedo plane, speed is the ace of spades. IF (big if) the max speed of the TBM allowed to to outrun it's fighter opposition, I'm right there with you. Since the speed upgrade DOES NOT allow it to outrun it's fighter opposition, personally, I give that much less weight than you seem to.
I have no idea what the climb rate upgrade is supposed to do. It's a significant upgrade, but for a third time, we're talking about the tallest midget. If the TBM could match a Zero in climb... ok, I jive with ya. It can't. But that doesn't really matter since your Kate substitution still gives the Allies a big upgrade. I'd call that a push even if it were a factor.
Armament, an upgrade in bore size, and another taller midget. I actually shot down a Zero from the back of Badcore's SBD in frame three and I felt like I won the frickin' lottery. In my experience, the guns on these Navy bombers are laughable and only there to make you feel like you tried.
Now, the dark side to what you propose is making the TBD substitute a flying lighter, i.e., the Kate. You would take away a nose gun and give a significant armor downgrade all in the name of speed. Speed, that again, will not allow it to outrun the fighter opposition tasked with shooting it down.