If it is already working, and your CAC is essentially $0 for those referrals, do you anticipate seeing a huge amount of growth if you introduced a referral program? One that would be large enough to outweigh the new costs?

Seems like a great thing to have customers loving you enough to be bringing you new leads without any incentivization. Perhaps you could find other ways of thanking your good referrers in a more natural way (even sending them a box of chocolates or a personal outreach from the CEO to offer a one-time consultation session) that might go even further, be more flexible, and keep you from introducing new costs and problems.

We are already pretty ok at gifting back (our hoodies/gifts have a pretty good rep).

I kinda default to “don’t mess with it” but sometimes wish we could do more to say thanks.