Ho hum, it seems the EMGS lacks the usual GWR fans so no responses to my plea! Anyway, here is a précis of my experiences.
If, unlike me, you have no plans to: use the current collection facility; fill in the horrible floor holes; paint over the bright floor; populate the interior; then the bogies can simply be demounted by unscrewing the central screws. I recommend this anyway as working with the entire chassis I managed to knock off one of the delicate end steps; minimise handling if you can!
The Bachmann axles are quite a bit shorter than standard so as to fit between their copper (supposedly bronze but looks like copper to me) pinpoint pick-ups. To fit standard EM pinpoint axles these copper pick-up extensions must be bent back or removed. I failed to do this adequately and discovered some wheel flanges were scraped by the copper remnants. The brakes annoyingly line up with the EM wheel flanges. After numerous frustrating attempts at reshaping the brakes (the linkages are horribly springy), the only way I could obtain reasonable running was to cut off the brake shoes totally. This is not especially noticeable because the shoes are mostly hidden by the bogie frame. At some time in the future I may add some cosmetics if the absence of shoes annoys me. I thought that opening up the axle-boxes slightly with a bearing reamer might yield better running. However the one axle subjected to this now runs less freely; maybe I overdid it. The proper engineering solution would be to drill out the axle boxes and insert brass pinpoint bearings. I reckon I would make a mess of that so this will only be performed as a last resort; new i.e. alternative bogies might be preferable.
There is a trick to getting the EM axles into (and out of) these bogies. Trying to insert the axle first time in one go is nigh impossible. Instead, insert one of the axle pinpoints into the gap below the axle-box, not into the axle-box, then insert the opposite pinpoint into its axle-box; the remaining pinpoint can then be pushed up into its axle-box. Be careful you do not squeeze the wheels together strongly or you will reduce the back-to-back.
As you can see, I added DCC Concepts pick-up springs to the axles and soldered the ends to the existing copper plates. Initially these naturally bunched against one wheel but I realised this creates drag so have tried to centre them on the axles. These springs do produce some friction but hopefully less than conventional pick-ups. Alternatively, there is space above the bogies for conventional wire or strip backscratcher (or even wheel-tread) pick-ups to be fitted simply to the copper uprights; this would have the advantage of current collection from all 4 bogie wheels instead of 2.
The electric coupling is by Viessmann; product code 5048, expensive at €18.50 but well-made. It has a very clever locking pin which can be lifted from underneath to uncouple. Its mate is fitted to a Bachmann 64xx which suffers from lousy power supply.