"Would you say that the Tungsten reducers are as effective as a similar sized mercury reducer? "

Yes, much the same. The tungsten bead one I had was made by Blaser and the mercury ones are made by (Sabi Rifles) the rifle buider.

FWIW I hunted a couple of years ago with a client & friend who had a .458 Lott built (if I remember rightly) by Dakota and it had 3 mercury tubes fitted..... he used to shoot full loads off sticks with that rifle single handed.......