Ghumbs, It's a hard thing to quantify what makes a turntable ideal. Numbers/specs are only part of it, and while I do find them to be valuable information to base certain things on, it doesn't always tell the whole story.
Ideally, a good table is going to provide a good amount of isolation from outside interference, low bearing noise, low wow and flutter, low rumble, good speed stability, and things harder to grasp, such as an engaging presentation, perhaps PRAT (pace/rhythm/timing), ability to generate a wide and believable soundstage/depth, and a general sense of a hard to call scientific term, musicality.
Back to your original topic, re: Pro-Ject arm on a Technics- I do believe the Technics has great specs, and fortunately for this argument can back this up, but falters, in general, trying to convey "musicality", sounds a bit "dark", and is not as resolving/involving as some other tables.
Where does this problem lie? Well, if we are to believe tests done, it's issues in arm resonance. If we are to believe owners, there's also the arm wire/path/RCAs.
There may be other factors, such as plinth itself or perhaps at some point even the PSU or bearing are limiting the table- but the main culprit is indeed the arm/wiring, and by replacing this, one mitigates the problem of the table.
Is this the best choice? Perhaps for some. It was for me (somewhat- I run a Technics SP25 and EPA 100 and 250 arm/ADC LMF arm/Pioneer PA-1000 Series 20 Carbon Fibre arm).
But one could also address the weaker or lesser spots on any given Rega or Pro-Ject and also end up with a fantastic table.
What's a person to do?
Depends on the person. The money spent on upgrades of the Technics SL1200MK2, or the Regas and Pro-Jects, can add up- into the territory of some hefty competition, like a VPI Classic, a proven winner right out of the box.
Others may opt the classic Thorens idler drive, the TD-124, and a 12" SME 3012 arm.
Some may go a Rega P5 with mods, or stock P7.
Or a modded Gerrard. Or a custom Empire- I love RedBoy's custom Empire, and would be proud calling it my own.
Still, others may choose the venerable Technics SL1200MK2, and modify it to lofty performance.
The Technics motor unit and plinth is ripe for modding- it's base performance is impressive, and a slew of aftermarket mods are readily available to pry your hard-earned cash from your pockets.
The stock arm can be modded to perform much better, but yes, I love the idea of the Pro-Ject arm on this deck.
Other popular mods are a new center bearing with better surface tolerances. And a power supply unit that allows plenty of juice as to never starve the table, which can occur under less than ideal power source conditions (this seems to be for most tables however). Fancy feet to further isolate the table has proven popular.
If you were asking me, I'd say consider all the above, but I'd also heartily egg you on, and say "Go for it!"
One last thing- theres also the elusive "synergy" of a system. From the arm to the plinth needing to match (some folk say the way the Rega arm dissipates resonance is not ideal for the Technics plinth- others may argue this), to the proper cartridge, to phono stage, to speakers and amplification- all play a big factor too, and it's not always easy to component match.
Then, there's also your room/environment. A whole other can of worms, a whole additional factor to consider- room treatments, speaker placement, etc.
What's a budding audiophile to do? Go nuts? Perhaps! But I say experiment, try it out for yourself and see what works for YOU.
Listening to others will give you some good foundation to lay your work upon, but in the end, the searching, tweaking and self-experimenting will net you the final answer.
I wish you well on this journey!
Mine is big black and clumsy and a good friend.
