LCDOUT is a synchronous command, so it is not affected by interrupts in either 4-bit or 8-bit modes.

8-bit is a little faster than 4-bit, but not much. The difference is a couple of instruction cycles.
After each data byte there is a ~50uS pause, and after commands there is a ~2mS pause, so those couple of instruction cycles are not noticable.

In a noisy environment with long wires going to the LCD, it's possible that 4-bit mode can get out of synch with the two nibbles resulting in garbage being displayed until it's reset. 8-bit will not do that.

You can use interrupt driven bit-banged serial to a serial LCD to eliminate the problem with interrupts if the USART is in use.
An example of that for 18F's can be found here ... http://www.picbasic.co.uk/forum/show...3931#post53931
It works on any pin too.