That is where it gets messy as the law is rarely either clear or explicit; however, anything over the stated limits is a "lethal barrelled weapon" and, hence, by definition a 'firearm".
One interpretation would be that a full auto airsoft toy shooting over 1.3J is indeed a prohibited Section 5 firearm, with all the legal ramifications of that.
However, that will probably only be properly defined when someone gets arrested for possessing such a weapon and the case goes to court, probably followed by an appeal. Let's hope that never happens.