[Verjigorm]

If no one has done a study, there's no way to tell except by brute force statistical analysis. If you're interested in doing that, I can develop an experiment, but I usually just go with... The more mind hunters you have, the better kill rate you have, so if you have an S4 and an S3 mage, it's better to put your astral booster on the S3 to give him access to mind hunt rather than on the S4 to give him a +1 penetration bonus (in general). The question you're asking only matters if you're using Astral Boosters to boost penetration, which is inefficient because they're more expensive.

Now, if you're trying to MH an SC, the best method would be to boost penetration to maximum on your most powerful natural mind hunters. The Void Eye will always be better than adding another point of astral...

So if you're boosting Mind hunters, do it in this order:

1. Advance S3 to S4 to increase hunt volume with a Starshine Skullcap.
2. Advance S4 (natural) mind hunter with Void Eye.
3. Advance Void Eye Mind Hunter with Rune Smasher.
4. Advance S4/VE/RS Mind Hunter with Ring of Sorcery (usually if you have the Forge of Ancients, Forge Lord, or National Astral Forgers [Hinnom]).
5. Repeat steps 2 through 4 with your S3 + Skullcap hunters.

MA Arcoscephale, obviously, has the easiest time with this process.