Like the title says. Extra padding for recordings is for just to be safe and normally never needed. Why don't make it look like commercial? I have +1 pre and +5 post for recordings and when my kid watches his Paw Patrol virtual channel there is like 8 minutes of waiting between episodes. And thats too much for a 4 year old. 
Channels does not save the padding information. The padding numbers are used to know when to start and stop recording, but that is stored in the recording rule that queues the recording. Once it is recorded, that data is gone.
Padding could maybe be implied by comparing the length of the actual recording to the length of the program in the guide, but there still wouldn't be any way to know how much of that excess time was before the start and how much after. Additionally, a difference in actual duration versus the guide data could also be due to interruptions in the recording.
I like the concept, but can't see it working practically, especially when the last show just goes directly into the recorded show without a break. You might just end up with a bad marker that is meaningless and potentially takes you too far.
You might be better manually trimming those recordings to make them clean files for a virtual channel.