I attempted to counter an opponent's spell using Syncopate in the Omnsicience Draft event, using mana from the emblem and a treasure token for X=5, exiling my opponent's Sorceress's Schemes. The game automatically chose X=0 even though I had floating mana, enough to counter the spell as they only had four available. This directly lost me the game for my third loss. Syncopate and other X cost spells should give you the option to pay despite the Omniscience emblem. Otherwise they are broken and should not be draftable (except maybe choco comet I suppose).