You cant have a huge outdoor area in FPSC - the max world size does not meet the criteria of a huge world tag - but you can still have unacceptably low FPS. As FPSC is relatively erratic in its inefficiencies trial and error is the only way to be sure what you can achieve in any given map.
To be sure the more you have in it (over differing level heights particularly) - the more likely you are to have difficulty maintaining fps.
To achieve what you may like in many instances wont be possible so you will have to design to the engines current limitations until such time - if ever, improvements are made to various speed issues.
Dont expect to actually fill the full map world area with complex content particularly if your level is not completely indoors as you will never achieve it and still maintain anything like acceptable frame rates.
There is now technically no object map limit - it was fixed in V1. entities, static in particular can certainly be used in large numbers - High thinking enemies and alike are a big drain certainly and so too as you have mentioned there are certainly areas of a map that can still cause impossible lagg seemimgly when FPSC incorrectly calculates massive polygon counts when it appears that it should not. e.g. when there is little in actual view - this occurs mostly at extremities so the further you get from the world centre in any direction while viewing a scene the more likely it is to occur.
The only way you can actually find out where an fps slowdown will occur exactly is to actually build the world so theres no way around that - choosing to build a complex world therefore, you should be prepared to accept that you may not achieve any design plan for a particular level and that you may have to be flexible in that design. If you cant do that then FPSC may not be able to achieve what you want.
If you get unacceptable fps creep in while level building sometimes the additional work of changing a level design and rebuilding parts and moving placement of dynamic enties around in a spaced out manner will improve things - sometimes not. Unfortunately its a matter of trial and error in high content, complex levels especially if they include outdoor areas.
FPSC Dungeon crawlers should be OK - thus far I see little that slows down this kind of level if built with gameplay in a linear fashion as recommended - thats what FPSC was designed to create. Once you expose yourself to the great outdoors your fps difficulties will increase in complex levels.
