The skill pyramid requirement is quite a limitation, actually. It means that after the +1/+2 level you need 3 more specialties to get to +2/+3, and then 4 more to get to +3/+4. That's 4 levels of Refinement, and nothing to sneeze at. getting an equivalent bonus with Item Slots s about as expensve...but a purely offensive or Defensive bonus or one on a specific Rote is much cheaper.
Also, bear in mind Enchanted Items, which also use Item Sots and are pretty damn awesome.
Yeah. For example:
Offensive [Element] Control focus +4 = one point plus the free focus item slots. That's pretty cheap.
Getting a +4 in [Element] Control requires a total of 10 in specialization bonuses, so we're looking at a total cost of 5 even with our initial free spec. Of course, specialization bonuses don't distinguish between offense and defense, so they basically count as 2 focus items...
Specializations essentially get you a wider range of bonuses for the same price, but it's much easier to get just the bonuses that you want with focus items. Especially if you're not really worried about additional elements. After your first 2 specializations, the other ones will be coming from additional elements. Focus items let you specialize cheaply.
That being said, the bonuses stack, so you probably want both. A wizard has 2 points left over at submerged that he can spend on Refinement. He can have 6 points of focus with 1 specialization, 5 points of specialization (max specialization bonus =2) and 2 points of focus, or 3 points of specializations (2/1) and 4 in items.
The first one requires a Lore of 5 to get a maximum bonus of +6. It also has 8 total bonuses (since specializations count for offense and defense, unlike focus items). The second one peaks at a mere +4 bonus. It does offer 12 total in bonuses, but some of those will be in a different element/specialty - only 10 can apply at once. Of course, this spread does require only a Lore of 2. The final option peaks at 6 with a Lore of 4 (so it provides the same max bonus as number one more with less skill investment). It packs in 10 points in bonus, all of which can apply at once.
The mixed strategy generally dominates the focus item heavy strategy, since it has the same cap, needs less Lore to max out, and provides more bonuses in general. And it spikes higher than the specialization strategy while generally matching it in total applied bonuses at once. It definitely seems like a strong overall choice.
Focus items can be more useful if you're mixing thaumaturgy bonuses and evocation ones. Your specialization pyramid for evocations doesn't help with thaumaturgy specializations or vice versa, so it's harder to go both ways.