Not sure if this helps, but last week I bought an SCT X4 tuner with a custom tune from the states, and one of the options when I flashed the ECU was the speed limiter which was set on 105mph which I of course set to no limit.
Point being it may be an ECU/tune Restriction.
Note: Mine is an SS, wishing it was a ZL1
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2017 Camaro 2SS (Euro) , Red Tintcoat, Red/Black interior , NPP, MCC