The article in Plane and Pilot was very interesting. He and built a standard RV-4 and used it to tow gliders, (That could be the other -4 in the pictures.) although I don?t see a tow hook on the FG RV-4. He talked about the weight increase for the retractable gear RV-4 was something like 80 lbs and he had a heck of a time getting it certified in Germany. (We have it so easy here in the States!) The article talked about the mods to the spar and the stiffing he had to do to keep the wing from twisting, where he put the fuel tanks and the electric hydraulic pack. That guy really did a fine job of it, right down to the retractable tail wheel!
He also said the speed increase was only something like 4 kts. over his RV-4 FG
Maybe someone else who still has the article can fill in more details.
Hardly worth the work, if speed is what you are after, IMHO, but the look beyond description. What it needs now is some fake guns sticking out of the wings and maybe a Luftwaffe paint job!