For Micro SD i'd say the faster the better, the device will read / write to it as fast as it can then.
I can't recommend any adapters or what card works best unfortunately. I just have a 4gb Memory Stick Pro Due that I purchased back in 2007 when 4gb was still considered quite a lot of space.
The AKORD New Micro SD TF to Memory Stick Pro Duo Adapter on Amazon does seem to get a lot of positive reviews for use with the PSP though, with someone using a 128gb SDXC card in the PSP fine. At £2.95 hopefully its worth trying. I can't imagine the import fee on a £2.95 item been a lot (you wouldn't pay anything on such a cheap item here).
If i was going to try a Micro SD to Memory Stick Pro Duo adapter i'd probably give that ago and ensure I used a quality Micro SD card with it, don't buy any sort of SD card off Ebay, theirs just too many fakes around. If your buying from Amazon ensure the Memory Card is sold by Amazon too. I use
MyMemory for ant type of memory cards. From what I remember they will ship internationally too.