Hi,
Welcome to AOS. What's your first name and where are you from?
Well, since I own the same T/T as you (a MK5) and have had it both
fully KAB'd and (now) Time-step'd, fitted with a Jelco SA-750D (and also Isonoes), I'm probably qualified to comment...
With regard to the two PSUs, the time-step is a no-brainer upgrade from the KAB, bringing a greater sense of rhythm and drive, less noise, greater clarity and dynamics, and improved pitch and timing (amongst many other things) - all of which combine to give you much more insight into the music, allowing it to 'breathe' more freely and really unlocking the magic contained in those grooves.
The other significant factor is that, when fitting the off-board PSU, KAB retain the original unit underneath the platter, leaving a magnetic field surrounding the playing area traversed by the cartridge, and this has quite a detrimental effect on sound quality, causing hum in some circumstances, and giving an 'opaque' characteristic to music that when identified is quite unpleasant. Therefore, if you're using the KAB PSU, a free upgrade is to take the platter off and remove the original PSU and casing. The improvement in sound quality is easily heard!
I can go into more detail on the PSU issue should you wish if you outline the specific information required
As far as the KAB-modified stock tonearm goes (with fluid damper) - I also had mine Cardas-rewired; it is very good (fitted with a decent headshell like the Sumiko), but again, much like with the PSU, things move onto another level entrirely when substituting it for the likes of a Jelco (either the 250 or 750, depending on what cartridge you're using).
What holds the Technics arm back, even when KAB-modified, is that the armtube is quite resonant (as indeed most are), but not in a 'pleasant' or euphonic way. The main resonant arm mode gives a 'peaky, 'shouty' quality to the sound which is very obvious once something much better behaved (damped) in that respect is fitted such as a Jelco or SME.
Therefore, not only is distortion lowered quite significantly, but the higher quality bearings on the Jelco and tighter tolerances in terms of overall engineering, enable the cartridge to reveal so much more musical information it's frightening!
Again, I can go into more detail on this if you wish, and handle any specifics you may have.
Hopefully that should keep you going for now
Marco.