that is disappointing as i was thinking of getting a similar rig just for this game, just to experience a full park you spent weeks on in descent quality. sounds impossible. Your GTX1080 has the 8GB DDR5x dedicated ram right? A tech from frontier recommended i have 32GB ddr4 memory. i have not bought this yet but come from RCT3, and there i would just play it paused and or closed as i was building the park (at least 50% anyway). Can you play/build PC 'paused'? and couldn't you build smaller paths and Que lines to keep the amount of guests down and focusing on scenery and bigger rides?
Sure, you can play and do most things paused, but it's not always the action of the poeple that slow things down, it's also all the very detailed scenery that takes some serious GPU work.
I think the complaints on the FPS slowing down on huge parks is a difficult one for the developer to sort out. During alpha 3 and beta there were huge complaints of the small size of the park. So they increased it by almost 250%. So now people are building huge parks and expected them to run smooth as butter. They are using rocks instead of Terrain to build mountains, they are double stacking scenery to get every bit of detail out of it as possible and not expecting this to affect the frame rate. Really?
There were limits on blueprints during Alpha 3 that kept them under 1000 items. So they upped the limit to 2000 which still is getting complaints. But you start copying 2000 items around your park a couple of dozen times, you're bound to have problems. This is what is happening and all you need to do is read threads in the tech support and bug reporting forum to find people expecting too much.
There is an option in the park management for keep then number of guests under a certain number (like 2000).
I think the developer needs to do a bit of expectation management here. Either you get the details and the freedom to do want your want or they will have to limit everything to keep you from overdoing it . "not more than 100 special affects", No more than 75 attractions, Max limit of 10k items in your park. Those kinds of things will keep the frame rate up but could be game breaking.
A real world example: Many cars have governors on the engine speed due to the irresponsibility of a very few individuals who drive too fast and blame the manufacturer when things go wrong.
I am not saying there isn't room for optimization, but I think the expectations are a little high if you expect to max out a park and expect it to run super smooth on ultra settings.