Shifting the structure rating to Magic/2+net hits may be better, let's see (for the same 6/6 mage as above):
99% chance of getting 1 hit: structure 4 barrier stops all weapons with damage 2 or less (light pistols and the like)
95% chance of getting 2 hits: structure 5 barrier stops all weapons with damage 3 or less (heavy pistols)
61% chance of getting 4 hits: structure 7 barrier stops all weapons with damage 4 or less (shotguns, assault rifles)
18% chance of getting 6 hits: structure 9 barrier stops all weapons with damage 5 or less (rifles)
Let's take a look at a 5/5 Lonestar Combat Mage (pool drops to 10, they still get +3 structure from magic, because of rounding up)
98% chance of getting 1 hit: structure 4 barrier
90% chance of getting 2 hits: structure 5 barrier
44% chance of getting 4 hits: structure 7 barrier
7.7% chance of getting 6 hits: structure 9 barrier
This seems reasonable still
For the lolz let's also look at a 2/2 Terrafirst! Shaman (professional rating 0, pool is 4, +1 structure from magic)
80% chance of getting 1 hit: structure 2 barrier, will soak 1 damage off every attack
41% chance of getting 2 hits: structure 3 barrier, will stop damage 2 weapons
1.2% chance of getting 4 hits: structure 5 barrier, will stop damage 3 weapons
Which, you know, for one of the least threatening adveraries in the book isn't half bad.
Especially when you remember that that structure 3 barrier is not just stopping light pistols, but is also taking 2DV away from every other weapon (making heavy pistols DV1 and rifles DV2 or 3), so I think I'll be running with this rule at my table.