get them to breed later on perhaps with a white leghorn rooster i have.
What is your goal? If it's just curiosity, then of course there is no real substitute for doing it and seeing for yourself.
But if you want to raise a specific kind of chicken, someone might have already developed it, and you could save time and money by learning what kind it is and buying it.
Crossing extremes like Cornish Cross and Leghorns is likely to produce a middle type of chicken. Many middle types already exist, without any need for special management as they grow.
If i restrict their feed, will they have a slower growth and forage around the garden a bit more ?
I have read some anecdotes of Cornish Cross that foraged when their food was restricted. I have never read of them foraging any useful amount while other food was available. One example is the book Chicken Tractor by Andy Lee. He described using a portable pen, moving it to fresh grass each morning, and withholding feed for 1/2 to 1 hour. So the Cornish Cross chicks would eat the grass and scratch the ground a bit in that time, because they were hungry and eager for breakfast.
One problem is that Cornish Cross are usually butchered quite young (8 weeks or so). So they do not have much time to learn how to forage and what to eat. Brooding them on the ground will help a bit, but no 8-week chick is going to forage as efficiently as an adult chicken who has plenty of experience.
To some extent, being a really good meat bird is opposed to being a good forager. The "best" meat bird just eats and grows fast, without wasting energy on moving around much. That way produces the most meat for the least feed, which is what the commercial raisers want. Cornish Cross are strongly selected to do that. Trying to get them to do anything else is going to be difficult. Not impossible, but difficult.
will be experimenting with a few chicks placing them on a restricted regiment to try and get them to breed later on...
The batch that i will experiment on will be allowed to free range during the day so that they can get in exercise and hopefully develop a healthier cardiovascular system along with stronger bone structure.
Probably some will do a bit better than others at ranging and foraging.
If you want to keep a few for breeding, you may want to put legbands or other markers on each chick, and pay attention to which ones do better, so you can keep those ones.
If you restrict feed for the whole batch, you will stunt the growth of all of them (so you get less meat from each one). That might be the best way to pick some for breeding, but it's not making the best use of the meat-producing abilities of the ones you intend to eat.