I don't know a damn thing about KA trannies but doesn't one of them have mounting points on the bellhousing for the CAS. Obviously the speed sensors would also be different.
Both the bell housing and trans mount points are identical.
You care correct the s14 has a provision for an additional sensor that counts teeth on the flywheel for low rpm control of idle I believe.
S14 trans on an S13 no issues at all (since the extra sensor isn't used with the s13 ecu)
S13 trans on an S14 ECU, sometimes rougher idle, usually not much that people notice since most cars are pretty beat anyway.
Speed Sensors: I am 90% sure they are the same. Easy way to check is to pull out the speed sensor and look at the color of the plastic gear.
