I would understand that if the syncros across all the gears were a single set, but having dual syncros on gears 4-6, that's supposed to eliminate any engine speed/transmission speed offset to allow the gear to slide in, and not grind. I'm not in the habit of changing the method of driving to suit one car unless is a specialty car (Formula Enterprise class specifically). I've driven 5 other manual Z's and haven't experienced this (3 were 'before I owned my Z' test drives and didn't drive it in the manner I would normally). The 2 I drove over the weekend were both MT Zs with sports pkg. A 2010 and a 2012, one with 42K and one with 23.5K miles. I could not replicate the problem on either of the two.
Coming onto a freeway, getting up to 55-60 in 2nd, then climbing every gear :3rd, 4th, 5th, to OD would slow me back down to 40-45 and I would end up downshifting again to get back to freeway speeds.
I'm hearing you say, "Oy! Don't accelerate hard in your sportscar (HA HA RACE CAR)
. You'll have to downshift after touching each gear to safely get to overdrive because all that speed you gained was just wasted while you're fiddling around with all the gears your car has in it."
18 vehicles owned, 9 manual cars/trucks. I've seen this only when sychros are crushed (1 car and 1 truck) or the throwout bearings was literally coming apart or an arm broke off(2 cars).
I do trust your judgement, however I cannot imagine grinding from 3rd to 5th is acceptable. If I had video with sound, I'm certain you would agree it's not right.