Yes, it was me asking a similar question some weeks ago. I was also in PT5 Presales mode.
In the meantime, I bought PT5-Stage, and I using it daily wit my MacBook Pro.
I have a slightly mor powerful MacBook Pro (i7 2,8 Gz, 16GM RAM, 1TB SSD), I'm no using any audio interface.
I haven't seen CPU consumption being above 22%, but I'm not a brilliant player!
So, for input I don't think you need any Audio Interface. For output Im using KEF monitores connected via USB They have 96.000Hz sampling rate, so I can achieve a latency of 2,7ms with no cracking.
I using two Roland digital pianos (F-120R and RD-64), both with MIDI over USB.
Weather your headphone is loud enough on intern MacBook Pro headphone plug might depend on the impedance of your headphone and your "needs". There are a lot of discussions (also here in this forum) whether in this case a headphone am or an audio interface would be the better choice. I haven't tested as my I'm fine with my BOSE headphone. I would suggest to try different headphones first.
I'm a fan of "sleek" installations, so if there is no real argument (i.e. a HUGE improvement) with an audio Interface I would rather invest the money for a better headphone or better speakers.
I hope this is helpful.
Thomas
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PT5.1 Pro • D4 • K2 • U2 • MacBook Pro i7 2,8GHz 16GB/1TB • Roland F120R/RD64 • KEF X300A • DT990Pro