I was able to duplicate these results on 2 units by paralleling a 2.2k across the resistor between VFB and Vout for each TPS53355 switcher (8 per board). That's all I did, I didn't upgrade my firmware or change voltage settings in firmware (not available in mine and read that the setting doesn't do anything anyway). I tested efficiency at different frequencies and the best results I achieved are:

Unit 1: 0.645 J/Gh @ 193 MHz
Unit 2: 0.625 J/Gh @ 175 MHz

At $0.13/kWh and current BTC price I'm profitable again, but just barely.

I have 3 more S3+ units to convert and did notice that one of the boards is a different (newer) version. I don't see the TPS53355 switcher on it or a voltage divider circuit that I can modify to change the hashing chips voltage. Any info on the newer board appreciated. I can post pictures of mod and detailed drawing if anyone is interested.