I used blue leds in my cluster but did not remove the green film. The final effect is certainly blue, not turquoise as you might expect, but it's not really bright enough. I like your idea of extra backfill leds, good strategy to avoid hotspots. I'm guessing you used a plain bulb to light the area behind the redline? IIRC that one also lights the needles via a light pipe.
One success I did have was soldering a couple of wires and an extra blue led to point at the odometer. That works really well.