Johnny: I didn't have any intentions of modding it if I ever pick one up, But what differences have you noticed after the mods? (I mean, they essentially double the price of the whole turn table.)Nope, I didn't double the cost of my turntable. I have a few low-cost aftermarket tweaks and one genuine modification.
By starting with an SL1210 M5G, I avoided paying for a tonearm rewire. That I haggled the purchase price down to $500 (at the local Guitar Center), which made it an even sweeter starting point.
My add-ons and tweaks are:
o Used Oracle Groove Isolator sorbothane (not gel) mat: $10
o Threaded brass cone feet from PartsExpress: $20
o 1-1/2" thick butcher block cutting board to platform the 'table on: $25 from Ikea
o Sumiko headshell (a synergistic match with the Denon DL-160 cart and brings the arm's effective mass up to a better match with med-compliance carts rated at 10-16
My one mod is the KAB damping trough at $150, and it took 30 minutes to install, taking my time and being fastidiously careful.
So my total investment is $500+10+20+25+150=$705, which is just $6 more than the list price for an M5G.
As for doubling the price of the turntable (if you modded it to that extent), don't consider percentages; consider what you get at that price.
For example, start with KAB's SL1200 mk2 ($475) and add the outboard PSU ($250), threaded clamp ($150), tonearm rewire ($169), fluid damper ($150), and Isonoe footers ($175).
You're up to $1369. At that point, the question shouldn't be if the mods cost more than the original turntable. The question is whether the resulting unit is competitive with what you can buy for $1369. I'm in the camp that believes it's highly competitive, especially in the areas of s/n and speed accuracy. Others here would advocate for a Rega, Funk, or VPI at that price.