There is more to it than just "current testosterone levels" because (like you mention in your question at the end) biological birth sex as a male would likely afford the person with a larger frame, muscles/skeletal structure, bigger hands, longer legs, etc.

And to answer your question: no I wouldn't have a problem with cis women competing with each other. Obviously, natural differences exist and set people apart. But sport and competition is predicated on a foundation of fair play, personal excellence, and mutual respect among participants.

Any artificial advantage or thumb on the scale, for whatever reason, undermines the integrity of the competition, devalues the effort of athletes, and erodes the spirit of the game.

Naturally occurring differences like extra testosterone, height, or flexibility might be advantages but they aren't unfair because they are inherent to the individual's unique physiology and not externally imposed or artificially enhanced. That's the line we've drawn for sport and competition for centuries.

So no, that wouldn't bother me at all.