On a track, where staying in boost is easy, I'd agree, but speed through roundabouts is often controlled by the guy in front of you, and exiting roundabouts in second gear can be lag, lag, lag, lag, and then off you go. Second is good for nearly 80mph in a 5 speed, and you're over 30-35mph before you're in boost, so the diesel you were hassling is 3 car lengths ahead of you before you hit boost.
But the diference is only 1-2-3 in 6 speed equals 1-2 in a 5 speed, and 4-5-6 in a 6 speed equals 3-4-5 in a 5 speed. For track work you'd just be using 3-4-5-6, instead of 2-3-4-5, so no more gear changing should be needed.
You can hardly drop it into first for roundabouts, and dual carriageways with roundabouts every 1/2 mile are common around here, and get used for some friendly performance testing against others.
But 5 speed boxes and drive shafts are not strong enough in my heavier car, and a 6 speed using diesel drive shafts is as strong as oem gets, so that's the way i've gone.