What sparkie missed was to point out that pumping those extra amps through the horn relay on the RCM is gonna make it go belly-up pretty quick ....

Definitely go with a free standing heavy duty 12v SPST relay, at least 50 amp rating, 100 would be better.
I'd also run 12 or 14 gauge copper stranded wire to the relay from B+, then to the horns.