The main supposed benefit from pressure fermentation is supressing ester formation, and this is usually with regards to lager yeast. I don't know if it has the same benefits with ale yeast, and more importantly, I'm not sure that it has the same effect on reducing fusel alcohol formation that it allegedly has on ester formation. Fusels may be more of an issue than esters when pushing fermentation temps on ales. What yeast strain are you using? In a porter you may actually want more of those esters anyways. Also, going from 65F to 71F fermentation temp is not going to knock a week off of your fermentation time.
I don't think you're likely to cause a lot of harm, so by all means try it out and let us know how it goes if you want. That said, I don't see a huge potential benefit.