Vis

Spending vis on spell casting is a good method of increasing the Casting Score for a single spell casting (see ArM5, page 82). A moderately experienced magus with a lot of vis to spend could easily get a Casting Score bonus of 20 to 40, depending on his Art Scores.

On the other hand, if the caster wants

to cast a group of spells at high Penetration (an Intangible Tunnel-style effect, and then several damaging effects through the tunnel), then using vis is unlikely to be a sensible course of action, as each spell will require vis.