Tiger Ace is not OP either, on two occasions have I encountered it and beaten it with relative ease (without SI). The trick is not to focus on the tank that much, and try to destroy everything else (no income = limited reinforcements). Unless your enemy is sitting on a heap of resources, it shouldn't take that long. chase infantry down, spread focus all over the map and avoid the tiger ace until it is hardly supported...
I only bought the extra commanders today because I wanted to have every commander and tactic available at my disposal, not because I felt like I couldn't win without them. I'm not going to say anything about the SI doctrine, because I don't know it well enough, but the Tiger Ace is beatable... In fact, as an Ostheer, it can be a gamble to order one in. No income can really come around and bite you in the *ss.
This being said, I don't like relic's approach on buying new commanders
Skins, maps or missions? No problem, but with commanders you will give more options (more options, not better options) to one player than you will to another.