For my money (and I'm an ex-SH, so don't really take my word for it) FB has the best view of the pitch of anyone, for calling plays; they can also have the biggest individual influence on a play by picking the right line at the right time, and coming onto the ball already at a full sprint.
For my money (etc etc) FB needs a base minimum of pace to be effective, but it's not really an advantage until there's a pretty big jump in speed. So yeah, I guess someone without that base minimum isn't really suited to FB, but you'd need pretty exceptional pace for it to start being a plus point, rather than no-a-minus point.
Of course, it's all opinion anyway.
For me, sacrificing defence from the FB is like sacrificing passing from a SH, game management from a FH, or lineout throwing from a hooker...
Every position has a prerequisite that is non-negotiable, and I won't sacrifice it (but then, we're not really talking about sacrifice here, but about compromise - Smiths defence is barely good enough at FH, nowhere near good enough at FB - but can easily be trained)