5.4 is fine for hop focused beers and that is what I aim for when brewing PAs and IPAs. The problem is that if I don't add a bit of lime to my water (its RO too), the mash pH will be too low and that echos into the kettle pH.
martin - I'm scratching my noggin here as well. I checked last night, and the water recipe I use for one of my house IPA recipes is strictly gypsum, calcium chloride and a bit of epsom (although i know you've been saying recently the latter may not be important).
this particular recipe has an estimated mash pH of 5.4, SRM of 6.4.
what gives - is your IPA a lot darker, hence needing the PL to get back up to 5.4? I should have looked at my other house recipe which is a bit darker to see what i do with that one, but I was rushing this morning before work and was looking up before i left in order to pose this question.