I don't think that it is possible to somehow match GT cars and R5. If GT's shall be competitive against R5 for example on our stages than the very same cars will be completely dominant in Spain as the roads are generally very different (and vice versa). Anyway 50 mm restrictor is nonsense. You have to consider that what matters about restrictor size is its area and that depends on diameter squared.

39 mm - 1194 mm2 - let's say 100%
42 mm - 1385 mm2 - 116%
50 mm - 1963 mm2 - 164%

As I said many times first of all I don't see any reason why to even try to push for GT cars when their manufacturers don't give a shit about rallying. For me it's complete nonsense to push for them against manufacturers who are actually active in rallying and who keep the sport going.