Attacker Barnett, 19, gave his brightest display in a Bucks shirt to date in a 30 minute appearance from the bench in defeat to leaders Bradford Park Avenue on Saturday.
He is yet to start a game for the New Bucks Head side but, despite his initial month-long loan finishing next week, Cowan believes the youngster can progress.
“He’s a very good player, that’s why I brought him in,” Cowan said of the Wellington-born youngster. “He’s extremely good in possession. “He’s got a lot to learn out of possession, but you can see he’s got another level to go to.
“He came on to take his opportunity and he’s impressed again, so it’s another chance for us to assess the team going into next week.”
Town academy graduates Ryan Sears, John McAtee and Barnett have spent time with the Bucks this term and Cowan believes such loans are ideal for development.
He added on links with former side Shrewsbury: “I’ve always been keen to try to establish a good relationship with them.
“They’re my old club and the reason I moved to Shropshire really.
“It’s important a club like theirs and a club like ours have a good relationship.
“I understand the rivalry, but we’re on completely different levels at the moment so there’s no need for that to come between anything.
“Whatever we can do to help them develop some of the younger players, like Ryan who has helped us, that’ll be great.”