NGC5363 / 5364 would look awesome in your 20 inch M&T! You must do it! I want to see it.
30 seconds... it part of an equation. Me being lazy + 8" F4 + tropics = 30 seconds. You see in 30 seconds I can get pics in cloud gaps. At F4 I can get subs in 30 seconds. The interval timer on my K-5 only goes to 30 seconds. In 30 seconds my thermal noise is low. In the tropics it's usually warm to hot even at night for 9 months of the year and limiting subs to 30 seconds helps keep noise down. Plus did I mention I'm lazy?
I have done guiding with subs to 10 minutes, but can rarely get that sort of sky time here due to cloud. This year we are in drought so it would have been possible...