I think streetable means just that, you can drive it on the street. As long as you can get a sticker and tags, that's all. I used to drive my 88 RX7 completely gutted and open header. It was annoyingly loud and not overly comfortable, but it got me to school, 30 miles each way, everyday... well... snow kinda sucked, but that was what the mustang was for...
I think it comes down to each individual of what they are willing to sacrifice. If they want to run a full race machine down the street, having to run a sprayer on the radiator and intercooler, dump additives into their fuel every stop and put up with the PD, it's streetable. A friend of mine has a mustang with, the last time I talked to him, was bored and stroked to a 614 (yes, six hundred fourteen cubic inches), got less than 4 mpg, couldn't run pump fuel to saves it's life, but was driven all over creation. To me, that isn't streetable, but he sure drove it.
Street-legal
is a different story, which should be the guideline, but who wants that? To that I will say a few years ago, I was watching a truck pull and then winner actually was a street-legal dodge cummins diesel. The owner even drove it home from the event. I think that says something.