5 minute frequencies of double-decker GO trains have a capacity equal to or better than Yonge if you remove most seats from the lower floor (keep ~4 for those with mobility issues). Commuters to the upper floor, inner-city travellers making short trips can stand.
Metrolinx needs to do a bit of construction to achieve that; but they already get really close on Lake Shore during peak hours (express + local together).
4 tracks (East/West around union) running at Hudson Tunnel -> Penn Station frequencies (2 min) has a theoretical total capacity around 360,000 pph landing in Union. Unloading that is a massive challenge but even another $5B tossed at Union + corridor is damn cheap capacity compared to subway extensions.
That's 3000 per train (standing level + upper seated level), 2 minute frequencies (90 pph per track) with full trains entering from both East + West on 4 tracks total.