You are actually a bit wrong. The OS R5 is their coldest surface racing plug.
I run a Vspec and a RG engines with 30% fuel. In my Vspec the P3 turbo plug is the only plug to use unless its over 100+* outside then you can try a P8 but the P3 is probably better.
As to the RG I use a A3 (medium hot) almost exclusively. It is almost the same as the A5 but is OS's sport plug and is cheaper in cost. The #8 plug is OS's hot standard plug. RG engines love this plug (A3) and they last a long time with proper tuning.
And yes, use only OS plugs in OS engines as the thread length is shorter than the other brands. If you cannot get OS plugs you can always use an extra copper washer on the standard length plug. Its not perfect but will work on giving you the close to proper clearance for the engine.
I have found that the McCoy #59 will work the best in a RG with the extra washer if you can't use a OS plug.