I have upgraded 2 of my 4 Tikkas, not because I think that the aluminum bottom metal is any better, but because the trigger guard openings on them are a bit bigger so I can fit a gloved finger in them more comfortably. I am still using factory mags on both of them.
I have used some of the aftermarket Tikka bottom metals for years and am now of the opinion that they are purely cosmetic. The factory bottom metals have metal pillars in them and I can torque them to whatever spec I need. The aftermarket ones are much better looking and feel better. But, the factory bottom metals work just fine.