Patches/title updates should work regardless of where the disc is from (based on personal experience having played NTSC discs on a PAL console).
The issue arises when you specifically downloading DLC (i.e. not title updates). The DLC you download ultimately needs to be compatible with the disc you have. For example, the Australian (Euro) PSN store wouldn't give me access to DLC that is compatible with an NTSC (USA) disc. Meaning I would have to either get a PAL region disc, or set up a USA account and fund it with network cards. This is just from personal experience though.