in the comics both John Walker and Lemar Hoskins are both juiced up , like Steve Rodgers was
kind of...........
they both got thier powers from the Power Broker , which was mentioned at the end of the episode
so far in the show , they don't have any super powers
but , I think that will change by the end of the series
here's what I think will happen , not a spoiler , just my thoughts
the bad guys , the Flag Smashers , we'll find out they they have a sympathetic cause , and Sam and Bucky will try and help them out
while John and Lemar will join forces with the Power Broker to try and take them down , , and in return will then get juiced up by the Power Broker
by the end of the series , John will give the shield back to Sam , and John will become US Agent and get his own shield , Lemar will be killed off and John will still be a dick
but , thats just a guess..........
as far as John and Sam being able to toss the shield around like it's a Frisbee , I'm guessing is because the shield is made out of Vibranium which gives it , it's own special characteristics