3 things:
1) Thread creep alert
2) Upgrade past? What does that mean - skip the 2" navigators in favor of a GTN 650/750? (Maybe it's a typo and/or maybe I'm not picking up what you're laying down. I assume you mean put in a 2" navigator - ala the GPS 175 or GNX 375 (if it needs a GPS and ADS-B).
3) Here here on the GNX 375. I absolutely love it. I bought my plane in November with stock 1979 avionics. Alternatives were a GTN 650 and GTX 345 (ADS-B in and out, bluetoothed to an iPad to get traffic/weather on Garmin Pilot / ForeFlight). Granted, the GTN has a slightly larger screen and a NAV/COM. (And even the Nav and or Com is optional!). But, for comparison, I got quotes for a GTN 650 and GTX 345 for $18 - 20k installed, depending on labor. I had the GNX 375 installed for $11k which included a new transponder antenna, cables, encoder, etc. I wanted everything to do with the transponder replaced. We are based in the DC SFRA and having surveillance crap out on us would be more than an inconvenience
My point is - I love the 375. Hindsight, we may have been better served with a 650/345. It keeps the boxes separate, so if for example, one craps out, we wouldn't be down the other. IE - 650 goes down, we can use our backup comm and still have a working transponder while we wait for repairs. Then again, if the GTX goes down, we're out for the count anyway.
For comparison, if we were to replace one of our legacy nav/coms with the GNC 255, that alone would be $4k plus labor.
Rambling here
What did I say about thread creep? Okay, time to bounce off the walls elsewhere and let the coffee dissipate
Cheers and again keep us posted.