SiteTitle is a convenience control, if you don't like the markup it renders you can remove it and put actual html markup in its place.
There are no h2 tags at the page title level, they are actually at the content instance title level, coming from the html content feature or any other feature you put on a page. If you click the settings link next to the title of any content instance you can on the General tab uncheck show title and it will leave out the title completely allowing you to put it in the html content yourself as you like.
Also in the latest release 18.104.22.168 (released today) you can override that h2 and use another tag if you want, just paste this into your user.config file
<add key="ModuleTitleTag" value="h2" />
and change the tag to what you want to use.
Hope it helps,