What's your water flow rate? Do you ever take baths? How many gallons does your toilet flush? What fixtures are being drained into OPs line? What's their water flow rate? Etc, etc. Yes, as you say, code is designed to make sure it works regardless of the situation, but nowhere in this thread is there enough information to determine whether something less than code is ok. Not to mention the challenges ever selling if a prospective buyer figures out this is an unpermitted bathroom that doesn't meet code.