Polistirex is weaker and it lasts longer. Not sure but I think the dose you're talking about is really quite high, I just googled and people seem to say it's about 75% as strong, so you're talking about 480mg + ~800mg which is actually quite dangerously high. And yes chlorphenidramine maleate is an antihistamine that has terrible effects at high dose. You must be talking about Coricidin which is well-known to be the most dangerous and harmful way to take DXM, and if it's 480mg from that, you're talking 2 boxes. Someone I went to high school with literally died from taking 2 boxes, JUST 2 boxes with nothing else. 1280mg of DXM alone is also enough to possibly kill you. What you suggest is a death wish and I hope you don't do this. I'm glad you posted here first though. Also you should never take Coricidin, it's really dumb and the high is way shittier than regular DXM.
If you take the 1080mg of polistirex by itself, you'll trip hard and that's a good high dose considering the other DXM doses you've posted about taking recently (I think that was you anyway taking in the 700mg range). If you've actually never taken a dose in that range before then definitely start a lot lower in case you're a poor metabolizer of DXM (I am, personally), in which case that dose could be quite dangerous.
EDIT: hang on, you already took it? Dude, what the fuck, why would you take it and THEN ask? Honestly I'm concerned for you... you should probably go to the hospital or at LEAST let someone know what you did so they can watch over you and take you to the hospital if/when things go bad. IMO you should try to throw up right now, it hasn't been long. You need to take this seriously right now and I hope if you make it through this that you learn something because you're gonna end up dead one day if it isn't from this, if you keep taking stupid doses of drugs without bothering to check whether you're overdosing first. FFS.