Against Leinster, the pair both caught Penney's eye, but eventually it was a late Brian O'Driscoll try, converted by Ian Madigan, that secured a 22-16 victory for Joe Schmidt's side.
"He is a bit special at what he does. He has come into his own in terms of gain line carrying but also is endeavouring to add other dimensions to it," Penney told the Irish Examiner.
"Like Casey he has taken time to adapt to the new environment but you want your big boys to come right at the end of the season and they are certainly two who have started to step up."
Defeat ended what slim hopes Munster had of reaching the RaboDirect PRO12 semi-finals but Penney was still pleased with his side's effort as a European semi-final with Clermont beckons.
"I couldn't have been prouder," added Penney. "Yes, we could go back and pick out some errors and some little turning point moments but the attitude was there, the commitment to the jersey and the province was there.
"It was just a tight game in which we didn't take some opportunities, they took theirs and that's the difference in those games."
Guinness PRO12 Munster News
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