Boosting assumes that X is a high level (70) toon that you want to use to run the others thru instances to power level them faster.

If thats the case then no - you dont want X associated in any way to the RAF accounts. The high level will trump any bonus to XP. The only thing you will get will be the mount and the A,B,C,D toons can grant levels to a "lower" level toon on X.

If you want to get the most out of boosting then you should create A - have A refer B, B refer C, and C refer D. That way the 4 alts get RAF bonuses and since the 70 isnt linked you can then use X to boost the others.