Mixed mode is not the answer. While it could provide, at most, 10% more slots, it will not address the congestion and delays.
Yes, the consensus appears to be that mixed mode on the existing runways would provide around 60,000 additional ATMs pa, which works out at about a 12% increase on current capacity.
And you are correct in that it would make little, if any, difference to delays or resilience.
Of course you could also argue that R3, too, would only have a limited, short-term effect on congestion, given what we're told is the scale of currently constrained demand for slots, and that traffic would grow in due course back to 99% of available capacity if allowed to.
Incidentally, R3 is planned to operate permanently in mixed mode from the outset, serving both T6 arrivals and departures.