There's no Limit of which I am aware, although there is an effective cap of +4 because any one attribute rating can only receive an augmentation bonus of up to +4. (See page 94.)
I'm not sure on that one. The power states that each hit adds 1 "up to your augmented Attribute maximum." In other discussions I believe it has been pointed out that this is the upper max limit, not just a limit of +4.
A human has a normal max STR of 6 and an augmented STR max of 10. Items such as cyberware/bioware cannot exceed a max augmentation bonus of +4, so if you only have a natural STR of 3 your augmentations can't get you above 7, but the "augmented Attribute maximum" is still 10 as you can increase your base STR by karma expenditure.
This means you could play a relatively low-strength adept who uses a high Edge score to add lots of dice to his Boost STR Attribute roll and potentially hit the upper limit for their metatype, all for a very very cheap 0.25pp for a single rank, no drain worth speaking of, and it would last even through a long and protracted fight (since duration is 2x hits in combat turns and most close-range fights are over very quickly).
Hmm..now I have to work on a character concept...
In terms of balance I think I might still prefer taking the hits granted by STR boost as extra dice on attacks with a STR-based damage code, but just having add directly to STR and therefore the DV of the attack would certainly be cool.
