I've had mine up to right at 150 near the top of 4th gear on the dyno and also while racing a z06, and it wasn't very stable, but I also had no intentions of turning at that speed, braking is always nice though. I have a 120mph speedo and it does wrap around but not all the way to the needle stop. Only reason I know it was near 150 is because on the dyno in 4th it was in the high 140's. I have bilstein hd, ipd springs, big bars, upper lower chassis braces, 245-50/16 rubber. It's scary and if I had to do it again I'd put much stiffer springs in the front and lower it down another inch or two for stability's sake.
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'84 wagon w/302 and T70 turbo 507whp/468tq

I haven't had mine higher than about 115 -- but it's quite anti-climatic. I've got full suspension mods - Bilsteins, IPD springs, poly-everywhere it's available, upper/lower front braces, big bars, adjustable torque arms and Panhard and V-rated 225-50-16 tires. Very stable and comfortable at speed.

I had my speedo recalibrated by the shop that advertises in Rolling magazine. It's very accurate for speed and odometer (they are calibrated separately). You probably won't hurt the speedo unless you wrap it around till the needle hits the 'stop' -- if you keep going past that, you've got a good shot at breaking something.

Get 'flyboy' to post - he road races his 347 powered 242 and has it at speed often.

Wind noise on the other hand is quite a problem. Bricks and all....