We did some much more informal testing on 243 and 425. First, we applied 243 and 425 to the surface of a strip of 1/4” poly, and let it dry for ~24 hours. We bent the sample strip and neither loctite significantly effected the poly. Next we heavily applied both loctites to #8 bolts tightened in a sample of 1/4” polycarb. In this case, the 243 created spiderwebbing, but the 425 didn’t. Based on this, I concluded that neither loctite would harm polycarb after the solvents flashed off, but if you put any stress (including tighting bolts) on the polycarb while the 243 is wet it can fracture easily.
We started this when we had polycarb links on our intake cracked at their mounting bolts using 243. When we switched to 425 we were able to assemble and use the intakes for a while without any cracking. The links do typically crack at the bolts eventually, but we have not tested a loctite free assembly to compare longevity.
TLDR: In our testing loctite 425 can be used with polycarb, but may cause minor weakening.