Display summary at the top of the table

One way to achieve this is to create a duplicate display and set the Summarized Table plugin settings to "Display the summary row only". Then insert this view into the header of your view using Header > Global (view area). You could then change your main view to a normal Views table display plugin (if you don't want summaries at the bottom, too).

Or create an identical attachment display with "Display the summary row only" and attach it to the top of your view.

Alternatively, you can copy views-summarize-views-tablesummarized.tpl.php from the module directory into your theme's templates folder and move the summary row to the top of the rendered table.

I'm going to add this feature, but in the 7.x-2.x branch. I played around with it a bit and it was not too difficult to do.

The commit adds two options to the Style options dialog, beneath the "Display the summary row only" option (now labeled "Views Summarize: Display the summary row only"): one for displaying the summary above the data (labeled "Views Summarize: Display the summary above the data"), and one for displaying it below the data (labeled "Views Summarize: Display the summary below the data"). (See the attached image.) The defaults are for displaying the summary below the data.

Could this be added to 8.x as well please?

Yes, I'm adding it to the new 2.0.x that I have yet to publish. I just need to finish getting the functionality to match what is in 7.x-2.x and clean up the code, then I'll release it. I got a little busy and haven't had the time to finish it up.