...and on the platform, mast height and the shape of the sail.
In technical words the ratio of picthing moment resistance and heeling moment resistance. Of course the spi must be flat enough to handle the higher apparent wind speed.

If the boat can handle one person in trapeze and one on the hull, than it certainly can handle one single person on the wire. From a pure spot point analyses, you should be faster. What makes us (including me) slow, is the time we spent in going in and out, the reduced spi trim capability, because one hand is not enough to trim fast and our mind set.
There is certainly a higher risk, since the sheet and the wire pulls the sailor forward, making him wary and slow.

I am certainly faster (vmg) while sitting on the hull and until I don't master going out, it would be stupid to do it.

From the points above the limited trim capability is the strongest contributor in my opinion.

It is summer time on the northern hemisphere and we all have no execuse to not try it.

Cheers,

Klaus