Always worth remembering that in the days before Hillsborough and the capacity restrictions that followed Maine Road held 52,500 (give or take) but rarely was it filled. Yet, no one bothered back then and in the seasons when City's average was about 31,000 every capacity game would then add about 1000 to City's average (simple maths - extra 21,000 divided by 21 League games = 1000 on top of the regular crowd). This is how MUFC managed to beat LFC for average attendances often in the 80s (and 70s). Anfield's capacity was about 40,000 and full often, while Old Trafford held almost 58,000 and some seasons crowds would range from about 30,000 to capacity, such as 1982-83 when MUFC attracted 30,227 for WHU and 57,334 for City and 57,397 for LFC. Utd's average that year was 41,552 and those games gave United an extra 800(approx) each on to their average attendance. Had United's capacity been 41,552 then their average would have been somewhat less than 40,000 due to the volume of games that attracted more than 40k. Liverpool used to argue they'd have a bigger average than Utd if Anfield had the same capacity and, to be frank, they were probably correct as they tended to have a greater average for the 'smaller' games.
So, if City's capacity was 65,000 but we were unable to sell out apart from say two games, at least an extra 1000 would be added to our current average (personally I think we would sell out PL games). Every 65,000 capacity game would add at least 500 to our current average.
For non capacity games - why not design any new 3rd tier in such a way that it gets shielded off for smaller matches. It's seeing empty seats that gets the Emptyhad jibes going, so why not have some kind of curtain/blind that simply shuts off the third tier from the time it's constructed. Then when a League Cup or FAC game suddenly attracts a crowd that needs the extra seats that can be reported as a positive story "this game is so popular City have opened their PL only third tier."
I see no logic in sticking with our current capacity simply because the Emptyhad jibes may increase for cup games. Build it and then work hard to make them come and see every increase as a step forward.