So previously "Wielding the Invisible Sling" was level 10 (Base 3, +1 to control stone, +2 Voice), and now it’s level 10 (Base 3, +1 for great speed, +2 Voice)? I’d rather have a new level 4 guideline “move a hard object fast enough to do +5 damage” to clarify things.
That is not what I said. I said that you can add a mag for great precision or speed.
It has been rather common to list a Base number which is actually a Base + Mags and happens at least a time or two in every published book. We can deduce the Base and how many points of Mags were added fairly easily (we did it a whole lot for the Fan Grimoire). What exactly the added Mags were for, if there are multiple possibilities, is not always a given.
I also pointed out that it is the one of the over a hundred Legacy Spells in the game, which have had the same TeFo, level, and fairly defined effect for at least three or four editions (some five editions). There are many spells which push against some extreme of over or under optimization to be at the legacy level. There are also a few cases where there is some “+ Mags for X effect” specifically to allow legacy spells to be the right level.
Without the fudging, many Legacy spells would end up higher or lower level that what they were historically. Generally higher, though a couple like Conjuring the Mystic Tower should actually reasonably be a Mag lower.
EDIT: I will add that the actually level is the least consistent for legacy spells, since there are several where 4th edition broke from earlier editions. 5th is pretty slavish in copying the 4th edition levels though.