rather than upgrading the license for license i would suggest you to go for a AP and sector antenna.
the sxt has very narrow beam hence u may not be able to cover a bigger area even if u upgrade the license.
Normis, it's not always the money you pay directly, which is important.
You also need to calculate the efforts needed to assemble a routerboard + outdoor case + sector antenna.
Or in the worst case to exchange an SXT by such a setup.
Upgrading a license from L3 to L4 is much easier and more effective then climbing up a tower and exchanging hardware!