I'd be happy for Sifa to play as many minutes as possible. We are losing 44 games between two impact middles from this year though, which will mean if Sifa moves our entire bench is pretty much different.
I don't however think Teig has a role to play from the bench. He just isn't that impact player. He's a great one on one defender and line runner and his strength is consistency over 80 minutes rather than impact.
To start next year we have the following forwards - AFB, Cam, Nikora (assume they all start), Kaufusi, Rudolf, Teig, Talakai, BHU, Tuks, Hazleton. So one has to miss out if everyone's fit, and that's before JC comes back. I'd assume pending BHU's/any injury Tuks misses out. That leaves us with a bench of Sifa/Teig, Rudolf, Hazleton, BHU. I'm not sure Teig/Toby coming on at lock will replicate what we got from Jack. I'm honestly not sure Sifa would though either.
Also given AFB should take more minutes from the starting role, there probably isn't Jack's 45 minutes available on the bench from this year. Which makes it likely that a bench Sifa plays <40 per game.
So I guess my opinion is given we are already 2 middles down from this year, when we already had games playing Berrell there. I wouldn't want to make it 3 by putting Sif full-time on the edge. I think we rely on the bench impact too much and would suffer once injuries hit. I do however think Fitz needs to find a way in which he's getting more minutes. He also needs to integrate more wide running for Sifa during good ball sets even when he plays in the middle.