Sluggish performance from time to time can be put down to any number of external factors.. a slightly crap batch of fuel, hotter temperatures outside the engine bay, caked filters etc. These things come and go.
My Cupra hits 60mph on the speedo in 2nd, and then I hit a very visible rev limitation (as intended). The trick is to hold the revs and pop into third at around 58mph as to not hit the electronic redline.
I'd say always bring 5th to as close to redline as you can. 6th is a cruising gear. It provides you with mpg increases and low-noise motorway travelling. If you're actively competing with the Cav GSi behind you or whatever, forget 6th if you can. I've found this to be the case, anyway.