Nice video and pix!
IIRC, those rubber balloons do a bit better if "seasoned" prior to inflation, warming them in an "oven", can't remember exactly, but maybe like 80-100deg. for several hours?
I think the highest soundings are made with the plastic envelopes. (Raven Industries, and here: http://www.aerostar.com/aerospace.htm, are examples) The limiting factor for the balloon's max. altitude is whenever it bursts; we used to cut a small vent in one of the seams of the plastic balloons, (wouldn't work with the rubber ones) the idea being as the envelope expands to full radius, the small vent slot would prevent a burst. (You probably knew all this stuff anyway.) Good luck, looks like a lot of fun!