That is, used in this way, simply means das heißt, as the original post speculates and as pointed out in #1 and #7. That is the easiest and most direct translation.
The difficulty in translating this particular sentence fragment, as pointed out in #9, is that vorausgesetzt (assuming) would be rendered redundant by das heißt, so in German, either vorausgesetzt gets replaced by wenn or that is does not get included:
"..assuming there is a spec, that is!"
das heißt, wenn es überhaupt eine spec gibt.
vorausgesetzt, dass es überhaupt eine spec gibt.