I haven't used Microphen with Tri-X but did use Acufine at ei800 with very pleasing results. (I used it 1+1 single shot because I was not comfortable with replenishment)(posted 8044 days ago)I would expect Microphen to be similar to the Acufine. You can expect to get contrast close to normal and sharp grain.
We tend to emphasise the actual effective speed of the film in these circumstances, but something just as important is your metering technique. In some low light situations the light is contrasty and you have to decide how important the shadows are. That's one reason that testing is essential.
Let us know how you get on.