I know you asked if anyone has tested with 30+ packages, which we haven't, but thought I would give you my results. I have another post in regards to this but in testing with 4.0, our login times are slower when attaching 1 or 3 packages compared to 2 AppStacks in 2.18. I only tested 2 AppStacks since we are new to App Volumes and all this was done in our testing environment. I don't know if real world usage can change all this as far as slower times in 4.0 compared to 2.18, but for now we are going live with 2.18.
When 3 packages were attached, our login time went to about 1 minute 20 seconds. In 2.18 with 2 AppStacks, our login was about ~30 seconds. Now our times were better with 4.0 when I had the Writable Volume enabled with Profile only. Login time then went to ~46 seconds. Attaching 3 packages kept the login to ~46 seconds with Writable Volume enabled. Even with ~46 second logins, it is still slower compared to 2.18. Also I read to avoid using Writable Volumes if at all possible, so that gives me hesitation on using them.
I am noticing similar login times as you are when comparing 2.18 appstacks to 4.0 packages. With just one 2.18 appstack, login time is ~45 seconds. With one 4.0 package, login time is ~85 seconds. These tests are with no writable volumes.
In my testing we could only get 20 AppStacks (maybe it was 19) before they stopped attaching. I didn't test logon times. I do think its intresting that it went from 13ish to 20ish limit (for us) but with the change with the "architect" or fomatting of AppStacks it almost implies there should be no limit or at least a very high one.