How good is the Technics 1200 Mk2 with all the KAb upgrades?

If you have all the KAB upgrades (fluid damper, isonoe feets, external power supply, rewired tonearm and good rca phono cable) already done, all you need is just to invest in the mid or high compliance MM/MI cartridge, mat and clamp (or use it with stock rubber mat without clamp). Vintage cartridges like Grace F9-F, Stanton 881 or higher models, Glanz 31 or higher models, Grado Gold, Technics EPC-205c mk4 ... they are all works fine with rewired stock tonearm, if the compliance is high fluid damper will help. I'm pretty sure with the right MM cartridge and all the upgrades this turntable works fine. In case you want to use low compliace MC cartridges you need to replace the tonearm (Jelco fits well and not expensive), but i wouldn't do that, high compliance MM on the stock arm works great. In my experience this turntable with upgrades is good with MM (it was designed for MM). If you want much better turntable just buy another one like the Technics SP-20 (i curently have one for sale) or SP10mk2 with ability to use custom plinth and any tonearm (including 12 inch tonearms). I would not use expensive tonearm on SL1200 mk2 turntable.    

There is nothing wrong with the stock tonearm and it’s much more versatile than Rega. Actually rewired stock tonearm is good, adding fluid damper is also good idea. Cartridge adjustment on the stock toneam is super easy because technics provided plastic overhand gauge and tonearm has VTA on the fly, counterweight is also easy to use (one the arm is balanced the VTF digits on the counterweight are spot on). This arm can be used with many headshells.

If you replaced the arm with Rega arm then you need a protractor to set up a cartridge, Dr. Feickert is the best protractor for reasonable price (a must have).

Jelco is better arm for Technics, but not the only one that can be used instead of the original.
Jelco effective mass is higher than stock Technics tonearm. 
Jelso is not designed for high compliance MM cartridges. 

Anyway, i think the Jelco alone cost nearly 50% of the brand new SL1200GR turntable which comes with a better arm and much better everything including thenew DD motor (all for $1500-1600 used).

I've never tried outboard PSU on my pair of 22 years old SL1210mkII (i'm the first owner), since i have upgraded everything myself (rewired them and added Isonoe footers) i only bought Fluid Dampers from KAB. I think it's much more important to invest in cartridge and phono stage first than in PSU. I ended up buying SP-10 mkII with different tonearms when i realized the limitation of the modded SL1210mkII. In my opinion it is more reasonable to buy another turntable when the SL1210mkII mods cost too much. Paid $1200 for SP-10 mkII in mint condition, fully original. The SL1210 mkII with all the mods can NOT compete with SP-10 mkII    

Well the you can hear the hum only at MAXIMUM volume which we don’t use in real life in the living room. Also in addition you’re hearing a hum from the phono stage and amp as you said.

Also your music is not at the "leadout groove" on the record, this is actually the end of the music and the record :)

If you want a better turntable buy SP-10 mkII used for $1200-1500 this is more than realistic price on the used market. I bought mine on ebay about 5-7 years ago. Then invest in a proper tonearm, custom plinth etc (that was mine with Reed 3p "12 toneam and custom teak wood plinth). This is another level of sound quality comapred to cheap SL1210 mkII even after all the mods. Or simply buy new SL1200G

I love Technics, but my favorite Direct Drive is Luxman PD-444