On the face of it I'd expect negotiations to change that maritime boundary, perhaps to somewhere between the pre- and post-1999 boundaries, or failing that some deal whereby the Scottish government accepts a greater share of debt in exchange for the more generous border. The thing about gas and oil is that the prices are volatile anyway so it can be risky to base too much policy on the money you will get from oil. It can go up as well as down, of course, but it would be something of a gamble and one that may not pay off.
Which, in a nutshell, sums up the problem with voting "yes", at least in the hopes of greater prosperity. It's a gamble where the risks/ rewards are unknown and unknowable. Better to base the argument on something closer to "we believe in smaller governments" (even in a world where the trend is in the other direction).
In turn this prompts another question. Why is Westminster worse for Scotland (where it has a representation of 59/650 seats = 9%) as compared to the EU, where Scotland currently has 6 seats out of 751 = 0.7%? Even as an independent Nation, Scotland's representation in the EU would be on a par with Slovakia's, i.e. not that much. Rather undemocratic, you might say, and yet apparently not.