I think it depends on the garage and how nit-picky they are, and maybe the specific setup of mods combined. When I rev my engine while stationary, during defloating my AFR usually goes towards the lean end. This is expected and fine. During emissions they always have to test at ~3000RPM while the car is standing still, and upon throttle release the combustion momentarily goes non-stoich, throwing the CO way over the limit momentarily. I think you could argue the <0.3 CO limit is only valid at idle and higher revs and not on throttle release, but the garage saw all my mods under my car and initially pinned it on "too heavily modded exhaust".
Didn't really matter much, I went there to see how they tested it, saw what happened, disconnected the defloater and asked them to go again, and then it passed. Likewise, if they let go of the throttle slowly instead of slamming it shut, it'll pass the test fine. It just scared me a bit to get that call saying "your car failed emissions" on my first check with my new engine and custom exhaust.. :P
Oh and I went with COP too - better be prepared for the future. Disconnecting it if you really need to is literally a one minute job.