How long does this take to execute

[tumbleweed]

I have to admit I haven't done this using PBP, but I don't see any reason why an 18F25K22 @ 64MHz using DT-INTS wouldn't work out just fine.
There's plenty of ram and speed available. For that matter, if you're just transmitting DMX you don't even need to use interrupts... they just make it easier to transmit one packet while generating the next one.

DMX calls for a fixed baudrate of 250K with 8 bits of data + 2 stop bits, so that's 11 bits total (START + 8 data + 2 STOP) for 44us/byte. Note that if you're using a pic uart to send data you'll have to generate the second stop bit using a delay of 4us between bytes, but that's not normally a problem.

DMX uses packets of 513 bytes (a START_CODE byte + 512 bytes of data). Normally packets are send continuously about 44 times/sec (22.7msecs) which is the max full-out time for 513 bytes, but as long as you send one packet every second that meets the spec. As the transmitter, there's no other byte-to-byte time limit so things are pretty flexible.

If you're the receiver then things are a bit trickier since you HAVE to assume that you could get 513 bytes at the full-on speed of one byte/44us. That's still doable with a 25K22, but you almost have to use RX interrupts to collect a packet. That said, I've done both transmitters and receivers without using interrupts at all. Sometimes they just complicate things more than they help.