On my 2005 PLUS 4, the ratio is 3.73:1 with the 5th gear you get 60 mph @ 2400 rpm inducing 57 hp. I found the revs too low and available power @ 130 km/h (80 mph) is relativly poor: 75 hp. If you want to overtake you need the 4th to get more acceleration.
Or whip it back to 3rd and let it really rip

Seriously; I think you have to consider 5th a cruising gear. I would always drop back one or two gears for fast overtaking, besides I like any excuse to hear the throttle bodies and exhaust note

The Mazda gearbox on the current Plus 4 is a pleasure to use so it is no real hardship to drop back a gear when you need and leave the top gear for relaxed cruising.