Resolution Audio Opus 21 or GNSC moddedd 50/55

After much cogitating and a recent chance audition of the RA Opus 21, I decided it is the player for me. However, I am well aware of the excellent work done by GNSC on modding the players. So the question, would I be better off with a stock Opus 21, or a reference modded RA 50 or 55? As a 2nd question, would the 50 or 55 be better. If they can be found second hand, the 2 options are about the same price.
Before anyone suggests it, I can't really go for the Opus 21 and mod it later. I live in the UK and the costs of sending a unit to the US and back+ local import taxes, would be prohibitive. I plan to buy in the US and have it modded before sending to England.
Has anyone had a chance of comparing the 2 options, or can comment on the benefits of a reference upgrade, I would be grateful?