Since TEXTREL is basically gone these days (for good reasons!), IFUNC is the same as any other call that is relocatable to a target not in the same DSO. Which is either a GOT or PLT, either of which ends up being an indirect call (or branch if the compiler feels like it and the PLT isn’t involved). Which is what the person you’re replying to said :)
A relocatable call within the same DSO can be a PC-relative relocation, which is not a relocation at all when you load the DSO and ends up as a plain PC-relative branch or call.
Sure, but they're already paying that cost for every non-static function anyway. Any DSO, or executable that allows function interposition, already pays.
Ideally you should just multiversion the topmost exported symbol, everything below that should either directly inlined, or, as the architecture variant is known statically by the compiler, variants and a direct call generated. I know at least GCC can do this variant generation for things like constant propagation over static function boundaries, so /assume/ it can do the same for other optimization variants like this, but admittedly haven't checked.