I'm not a nutritional expert, but I DO feed Strategy GX to my yearling. I used the Ultium Growth when he was a foal/weanling but quit buying it due to the price as well.
So now I am using the Stategy GX and he is blooming, BUT I only give him about a pound of it a day just as a treat and to mix his vitamin/mineral supplement in. So I'm not feeding it according to label directions.
What I do feed him is about 3 flakes of alfalfa a day, 2 flakes of Bermuda (so he pretty much always has something to munch on) and then his pound of Strategy GX and a vitamin/mineral supplement.
If I was feeding the GX according to label directions, the additional supplement would not be needed (and probably isn't anyway) but I basically decided to cut out most of the grain products and just give him a little bit as a treat along with a vitamin/mineral supplement just to make sure he isn't lacking anything.
I really think the most important part of the diet is high quality hay.
My baby is pretty high strung, so I really don't want to add a lot of grain products if I can help it. If your baby isn't a wild-man like mine, I'm sure the GX according to label directions will be great for him. It's made to be fed to all classes of horses and if I'm not mistaken has feeding instructions on the back of the bag for weanlings too.
(I'm not saying the GX makes him overly energetic, I'm sure it doesn't. He is energetic by nature and I just don't want to encourage him, lol! That's him in my avatar.)