The thing is, the Miata's suspension is double wishbone front and rear, and both control arms are mounted to the subframe. It's not really an issue of one end or the other flexing, it's 90% an issue of both ends not talking to each other because they're connected by a piece of paper.
Still, every little bit counts. If you triangulate the strut towers to each other and the firewall then you've boxed that section and greatly reduced any chance for deflection. If it's available, always get a one-piece welded strut brace that triangulates the firewall. I'd have gotten one but Mazdatrix's piece is like $350 and I didn't even find out about it until I already had mine (which brings the engine into the equation with a near-solid torque brace mounted to the strut brace).