The aluminized fiberglass is a good occasional protective shield. However it brings two limitations that you want to be attentive to. One is softening and the other is the oven effect it has over time on the encapsulated fiberglas core as a result of repeated heatings.
Aluminum melts at 1215˚F and softens at 600˚F. The manufacturer of the aluminized fiberglass that suggests it is useable up to a temperature of 1100˚F is providing optimistic, highly suspect guidance at the very least.
More significantly fiberglas resin that is repeatedly heated to high temperature (well below 1100˚F will experience resin deterioration that effectively reduces the resin, that bonds the fiberglass fibers together, to a talcum powder like residue. The bonding it provided to the fiberglass was long since lost.
Another consideration is the cooling of the heat shield. Aluminum is a heat conductor not a heat shield. While aluminum in sufficient volume is an excellent heat conductor. fiberglass is not. That means the heat the aluminum is exposed to is transferred to the fiberglass where it gets trapped and not surprisingly cooks the bonding resin yet again.
For the
very occasional shielding of heat, aluminized fiberglass can be an attractive alternative. For a regularly repeating exposure to 1000+˚F (exhaust gas temps) heat source you would be better served with a solid aluminum plate of at least 0.125" (3mm) that has some source of air flow passing over it to sink away the heat it has absorbed off the headers.
This is a link to a State side performance shop that sells the Lava Rock brand of heat shielding, click here =>
Winding Road Racing. If you do an internet search in your geographic area you will find local race shops that offer the same or similar products. When you embark on one of these sorts of missions supplier proximity can be a real advantage.
BTW there is an ever growing reservoir of credible providers offering a variety of heat shielding. Get the best one (from your evaluation) that is as geographically close to you as possible.
Ed