The gauge will go to 158, but the actual speed will still be 155. Most gauges now give you a 2-3mph buffer-zone. This is why my tC tops out on the gauge at 127, but is governed at 124 mph (FTL) Trust me I know, autobahn FTW!
That again like he said is quick acceleration up to 145mph, then it kind of levels off. But at that point, its pretty much just for numbers, not for the seat g's.