I went along to submit my marriage extension application last Friday at Chaeng Wattana. Whilst I was there, I asked 2 separate immigration officers if I can travel during the under-consideration period as my current extension of stay and multiple re-entry permit expire on 11th April and that's the day I collect my new extension, they both confirmed that as long as I'm back before 11th April then I can travel but when I reenter I will only be given a stamp on stay based up to 11th April so when I return on 5th April they will give me 6 day stamp date. Applying early so that the under consideration is within your current extension validity seems to allow you to travel with a valid re-entry permit.