I used to own a modded DR-70D, modded according to Jim Williams' specs. Cheesecadet did the same, and I used the same shop he recommended to mod mine.
It made an improvement, but in retrospect I wouldn't do it again. No disrespect to any modders, but I think these mods (Busman, Oade, etc.) are no longer worth the incremental improvements they make for the cost, because we now have reasonably-priced professional-spec recorders available. The mods may replace opamps and caps, but they aren't going to do a major overhaul to the input stage or install a better ADC.
If you want something better than a 70D that does 4+ channels, sell it and put the money towards a Zoom F6 or Sound Devices MixPre-II 6. The step up in quality is not small, especially in self-noise (a particular weakness of the 70D). If you only need 2 channels, get a Zoom F3.