Air leaks with slip fit collectors and EFI causing rich A/F?

[C5GTO]

I am in the middle of fabricating headers for my Miura project car and was warned by someone that single slip merge collectors can leak air in causing a faulty O2 sensor reading that leads to a rich A/F condition. So my question out to others that have a similar setup (EFI and equal length headers with single slip collectors) is if you've experienced an issue with A/F mixture rich condition. If so, were you able to permanently resolve/mitigate the issue? If so, how? If you have this type setup and haven't had an issue, I'd like to get that feedback as well.

My quandary is that I already have had single slip collectors made and was told by the very reputable shop that built them that they should not leak if the primary pipes going into them are round, properly sized and properly secured with lugs/bolts. So I have professional header people telling me what I have should be OK and another person that's very adamant that I'm sure to run into an issue. On one hand, I don't want to go to a bunch of time and expense to solve for something that has a low probability to be an issue and on the other hand I'd hate to proceed forward only to find out later that I need to re-engineer and re-build the headers.

I'd like to get a larger data set of people's experience if I can. Please pitch in if you have experience with this type setup.