Ok a lot of 15 sided posts in a wide open area. No way its going to work. Sorry
Vis time is only your first and least problem. Your wpoly is also going to go through the roof (of course this depends on how big the area and how much stuff is in it but most likely you are going to have serious wpoly problems)
I am usually carefull not to use multisided posts in an open area. I would make them 4 sided and use a texture to give them a beam like look. I would also be carfull about how many you have in there.
If you must have more than four sides I would keep to the minimum number of sides you can (I would try 5 to see if that worked if not then I wouldn't go over 8). Don't forget you might be able to get them to look cool by finding the right texture to put on them. It might not be exactly the look you originaly wanted but you might find something that looks good anyway. This is part of making a level. You just can't always do the stuff you really want to do. You could do it anyway but then the level becomes unplayable.
I would suggest you read the doc "Poly count reduction methods" by David Hyde located at the rust site.
Also read the thread with the subject header r_speeds initiated by tatuone in this forum.
You could also read my design philo for Nukesville II:
(Oh my god I did it again I pimped my own level and shamelessly plugged my website
Seriously the above reading will help you get a foundation for trying outdoor areas.
As for making the posts hollow that would probably be a bad idea as then you would have bunch of brushes all of which touch eack other and that would cause bsp to fragment them into a bunch of polies and you would get a higher wpoly count than a solid brush.
Also I could have sworn I typed something up recently within the past week or so that gave a brief overview of doing outdoor areas. I thought it was in this forum but it might have been on ICQ. Oh well if you spot it in this forum it might be a good read as well if not the other stuff I gave you will give you the info you need
Hmmm I must have read this post then went away for a while before answering it cause Eutectic hadnt posted when I first read it. I didnt realize I had been gone that long theres is ahalf an hour difference in our posts. Anyway you can see he recomended the article by David Hyde as well so you know its a must read
[This message has been edited by claudec (edited 10-12-99).]