I think you're under-costing the effects of those features.
Terror flank charges are a strong possibility with a griffon, and against a fair number of armies that's going to be a heavy tactical factor. The mobility from flight is nothing to sneeze at either, and opens up a tremendous amount of options for attack. Considering you can also bring a stank (or two!) and split enemy artillery, you're left with a monster with some good CC ability, mobility that can wreak havoc with tactics, and terror (which is always a potent force).
Now, it is indeed overpriced currently, no arguments on that. But, I think 150 or so is definitely the right area. The War Altar does have a lot going for it, but it's not as mobile and while it's a strong unit, it doesn't necessarily lend the same tactical punch as a flying monster. It's truly the superior choice at the moment, and that's what we need to fix. At 160 for a griffon or 100 for an altar, you have a real tough decision to make. Under the current pricing, the altar stands out as a much smarter buy.