I'm still digesting CF's crunch (I see big changes on the horizon for my SRM character--lots of Working for the Man), but I was thinking along the same lines. I would probably take Reakt before I took Cerebellum Boosters (no need to spend karma on a PQ, cheaper than Rating 2 CBs) and I'd also optimize the Reaction Enhancers.
Boosted + Rating 3 REs (used) + Optimization + Reakt = +4 Reaction, +1d6 Initiative, net +7 dice to Defense Tests (Jesus...).
Total Cost: 116,050 nuyen, 2.652 Essence. It's probably worth the investment to upgrade your boosted reflexes to alpha.
For me, the magic number when rolling +2d6 initiative dice is 14. If your post-augmentation Initiative attribute is at least 14, you'll get the third initiative pass more often than not, 53.3% of the time. Going to 15 bumps you to 72.2% (18.9% delta), 16 is 83.3 (11.1% delta), and so on. You can figure out exactly how much diminishing returns with which you're comfortable.