I should think you'd be all right without removing things so fast... you can always finish it up and then get some people with lower-end machines to beta test it for you and see if you get any lag..
it would fairly easy to go back and remove objects at the end to reduce lag then to try and take it into account now perhaps and end up settling on the premade forest pieces.
If you're around 18,000 I'll bet you'll be ok with just going ahead and placing those roads and buildings and such. On my old computer I was able to play all the CwC islands without a problem except for Malden sometimes lagged. I'm betting that as long as you steer clear of the 100,000 mark you won't get too much lag on low-end systems.
I've got about 12,000 or so on my island now, and predict to finish with about 18-20,000 objects
and with every object placed by my own hand