If bards can keep their normal casting time, then suddenly these rules make them the ultimate party nukers! They don't have the 1 round casting time penalty, no damage cap, double damage... Sure they don't get many nuke spells, but they are strictly superior in every way compared to full casters now.
Also, they still pick up a few good spells (like Glitterdust) at about the same time that full casters do. So as low level casters, they might come off as a bit stronger, but fewer times per day.
As they gain levels, the spells that other casters pick up will out pace the bard quickly, so it becomes a balancing act of fewer spells of lower level as standard actions vs better spells and more spells at a much slower casting time.
At the table, the effect I'd expect to see is full casters finding ways to hide themselves or otherwise set up ambushes and try to drop their best, most debilitating spell at the onset of combat, to make future castings easier. For example, setting up with Invisibility and/or flight and opening with Solid Fog/Black Tentacles would probably be optimal. If your opponents stay tied up for 2-3 rounds, you can cast your other spells with impunity (or close to it).