I would be pretty close to 20% if I migrated today, sounds promising.
I've used Disk Inventory X which seems very similar to GrandPerspective. Lately, I started using OmniDiskSweeper which I like even more. Both of these helped me to get from 200 to 93GB.
I'm pretty sure I killed hibernatemode a while back. And /var is only 4 kB.
Well, it sounds like you've got a good handle on things! I would say you're good to go then. You just need to avoid filling up too much of that empty space since there isn't any TRIM in OSX.
My understanding is that blocks in an ssd cannot be overwritten - once written to, they have to be erased before they can be written to again. The issues comes that when you delete files from a system without TRIM, these blocks are not erased. The drive will attempt to use all of the free, unwritten blocks first when writing files, but eventually they will run out. At this point, the blocks that are free but haven't been erased have to be used. This is what you want to avoid, since now writes will require the block to be erased first, and then written, slowing things down. The idea of TRIM is to erase these blocks when the file is deleted (or at a convenient time when the drive is idle) to avoid having a drive full of these "free but full" blocks.
So, if you have extra space, you can delay the point at which this would happen. Controllers have a degree of garbage collection to also help with this issue. The drives in the new macbook airs have very aggressive garbage collection to make up for OSX lack of TRIM - if you haven't bought a drive yet, the Kingston SSDNow V+100 series uses this controller.
The aggressive garbage collection can help retain performance as the drive becomes cluttered, but can potentially reduce longevity. Flash memory can only be written to so many times before it goes bad, so a lot of garbage collection means extra writes -> reduced lifespan. How much less of a lifespan and if it's something to worry about is something i'm not sure about.
I've definitely left a wall of text here, but I've got one more bit of advice to configure your system for the ssd, and it's to enable 'noatime'. Unix based systems normally keep track of the last access time of each file, or atime. This timestamp is modified every time a file is read! Imagine if spotlight is indexing your drive and reading every single file - that's going to cause a lot of extra writes as the atime values are changed! Thankfully, atime can be turned off. It's called noatime, and there's a good guide on how to do it here:
http://blogs.nullvision.com/?p=275
There's also some other SSD advice there, but I wouldn't bother with the RAM disk thing, as I've read conflicting info about how good of an idea it is.
