Yeah doesn't seem like you're hitting a ceiling. If you want to increase threshold I don't think its vo2 work you really need. It wont hurt short term though.
I've never done vo2 work to build threshold. Hell I never did it to build vo2. You don't need to ride at vo2 to build power at that duration. The only reason I did it was for repetability.
The best thing I ever did for 5 min power was riding a lot. And genetics, which mean it's always been decent.