That was my thinking pal.
But there is no hard and fast rules with breeding from hybrids so will be interesting when my eggs hatch.
Even my Galic x Galic won't breed exact Galic.
If you like the sound of the Greybro I would give them a go as a meat batch, then at about 12 weeks if any look a little lighter with better movement keep them back, give them as much free range as possible and don't overfeed, just see how they go and worst case scenario you can always eat them

I think Piggotts do a slower growth bird as well.
You could keep a slower growth cockerel to put over the Greybro Pullets, similar to my project.
But I think Crosslings any hybrid to a pure breed should reduce the exaggerations and make for healthier birds, the trick will be not to reduce them too much.