Is overnominating actually a problem under STV? Assuming your candidates' preferences transfer well, it shouldn't make a difference in the outcome (and if the transfers are poor, then surely it means the party would have done worse without them).
It’s a major problem! At any STV transfer you’re looking at a 5-15% leakage. When you’re transferring surpluses at reduced value that becomes basically nothing, but when you’re eliminating a candidate with votes at full it’s a major bleed.
Australia and Malta use countback to fill vacancies, which means parties have to run a full slate and eat the leakage. But Ireland uses co-option or by-elections which strongly incentivise running an exact number of candidates. Council elections in particular are strewn with examples of wild over and under-nomination.
A classic example is West Tyrone 2007. SDLP has 1.02 quotas of votes, but split between 3 candidates (0.45, 0.38, 0.18). They gained about ~0.15 on transfers, but lost about ~0.35 quotas on their own eliminations, with only about ~65% of their votes transferring in ticket. This is an extreme example, as there was an ex-SDLP independent in the race, but if they'd only run one candidate they might have got a slightly lower vote (0.90-95?) but their votes wouldn't have an opportunity to leak out the ticket, and they'd only gain votes from transfers and be guaranteed of election.