I'm not at all keen on the idea of effectively reopening St Enoch. It would increase capacity for heavy rail services, sure, but it would leave heavy rail responsible for transport needs which may well be much better suited to a light rail solution.
The Maryhill line is just not at all suited for heavy rail, as the distances involved are short and there is strong competition with buses. Even if more paths were available into Queen Street the Maryhill service would be the very last one to get them, as it requires a lot of subsidy and increasing the heavy rail frequency would probably require even more. A tram-train solution would allow the frequency to increase while reducing subsidy, as it would be a far more effective way of getting into the city centre. All those extra stops that it would have between the city centre and Ashfield means more passengers, and thus a reason for a higher frequency, and thus even more passengers. As I described on other posts about light rail, the service would then be able to do much more than carrying people to the city centre, and so passenger numbers and the benefits it would cause would be even greater.
On the south side, a Cathcart Circle conversion may mean a slightly lower standard of train than the 385s but it would allow effectively unlimited frequency increases and infrastructure changes to make the service even more useful and profitable. One such idea would be to build a street running section branching off at Whitecraigs and then following Ayr Road through Newton Mearns. You can have as shiny a heavy rail train as you like a few kilometres away at Patterton but a tram-train within walking distance will always be more appealing.
I think such a metro system would be totally complementary to a cross-city tunnel, and so the existence of one does not remove the case for the other. The St Enoch option, on the other hand, would try to do both and probably block both of them from happening. If you built a cross-city tunnel after reopening St Enoch, then you simply wouldn't need St Enoch any more. Likewise, if you did some of the line conversions you would probably end up using up the freed capacity by moving services away from St Enoch.