Had my first WRX for about a month now, and have had the running in oil change done.

I've noticed recently that it is quite sluggish when trying to accelerate in 5th on the motorway at aroun 70-80. Is this just comparing it to the kick you get at lower speeds/higher revs, or is it quite slow here?

I don't always fancy dropping it to 4th - it'd be nice to have a bit more power top end when motorway cruising.

Would PPP help here, or is that more focused on higher revs/lower speeds?

 

Your engine is only doing around 2700rpm at 70mph in top. If you're talking about an otherwise standard car, it's not developing a lot of boost at that sort of engine speed.

Changing down to fourth is probably the most practical option. The PPP certainly will help noticeably, but there are other ways of getting even more low down. Trouble is that all of these will invalidate your warranty.