No, they wont be reduced. I played a lot of quest warlock
I can second that. Lots of questlock games. No reduction sticks: neither Emperor nor the 0-cost due to quest hero power.
Some time ago they explained that any changes (buffs in minnions, cost reductions, etc) would only be kept while cards moved "forward on the natural cycle of gameplay", which is deck -> hand -> play.
This means a minion that is buffed in deck keeps the buff when draw and when played. But a buffed minnion that cames back to hand or to deck does not keep the buff. The same apllies for cost reductions in hand, when the cards move "backwards" to the deck.
The obvious exceptions are cards that state otherwise, like Kingsbane and that paladin dude.
There are certain cards that are reduced and go to the deck with the reduced cost but with Plot Twist it will return to its original mana cost. Also the minions with buff return to their original statistics, it happened to me with Hir'eek, the Bat with Soul Infusion :(