My models suggest that a TD10M would be less efficient than the Eminence Delta 10a spec'd by Wayne. This would require a crossover change for sure.
I'm curious if there are any upgrade drivers for the pi7 midhorn. I read your old midhorn thread and you mention the JBL 2012H, but it is no longer available. The JBL 2251J is very similar in spec's but uses a differential drive motor...and it is a 16 ohm driver. I'm curious if you would consider this a good fit for the horn if I were able to alter the crossover of the pi7 myself.