I just want to report back, that this issue is resolved (don't know if there's a way of editing the subject to - Fixed).
A lot has changed since I originally posted the topic - Veeam 9, Veeam 9u2, Veeam 9.5, an extra Proxy + Repository. We've changed block sizes, compression, database - all kinds.
Just retested this following the recent v9.5 upgrade, and the Instant Restore VM boot up time - which was >12 minutes to the logon prompt, is now just under 2 minutes.
Once logged in - everything seems a lot more responsive, too.
I'm not sure which change has resolved the issue - v9.5 is the obvious answer - but the previous performance issues were severe for v8/9.
I'd have thought the change to per-VM backup files was the most likely fix (we had 30x Forever Incremental Restore Point backup chains, with >50 VMs and several TB files), but the test above showed poor performance with a single backup file.
I'd still like the backup times of <1 minute mentioned above, but factoring in domain policies etc, which slow down our boot up times anyway, we're happy with the performance.
Advice to others is persevere and keep to the best practice - however long it takes to implement