Justine's serve may have more mph, but this year is hasn't helped her out at all, since she serves around 50% and tons of doubles. Of course, they say she is changing it, and once she gets used it, her serve will be better.
Anyhow, my point is, at this stage in her career, I don't think Martina will totally revamp her serve like Justine did, and this probably means that Martina's serve won't improve a whole lot.
However, is there any reason, why she couldn't work on developing more of a kick second serve? Maybe her motion doesn't allow for this, I don't know.