Just curious about the numbers you mentioned there - How many of those 800 legendaries and 3500 Heros are actually active on the forum? If you find that out, the numbers will drop down drastically.
If the main goal is to stop signature campaign spammer, then i have another idea such as :
1. Signature limitation based on your merit amount, not your rank
2. Signature only enabled for those who earned x amount of merit within y last month
I doubt signature campaign spam is that big of a problem these days. Very less bitcoin signature campaigns and strict moderation including merits have cut it down. Extreme amounts of spam come from the altcoin bounty campaigns including massive account farming. Some merit law should definitely be implemented in those sections.