I was a bit surprised by this, so will point it out:
My understanding is that the 150mm is measured with the fork tubes off the bike, collapsed, and empty (springs, spacers, washers NOT in the fork). That's how I did it, anyway.
If they're still on the bike (not collapsed), a/o the springs are still in them displacing oil volume, that number will be different. What it will be, I don't know.
IIRC, on other bikes I've owned (I've changed fork springs on all of them), there was one number for doing it "on the bike" and a different number "off the bike" (springs removed, collapsed). My memory is notoriously crap, tho, sorry.
I agree with your reasoning there but: What I actually measured was 175mm, springs out, front collapsed ON the bike. And I don't think it'd be different with the forks unbolted, if anything it'd be lower. O.K. I lost some oil when I extracted the springs, but I doubt 2.5cm worth.
I'd be a bit careful about trusting figures from 'the book' here. Suzuki have been fairly lax in getting the documentation exactly right in the past and have obviously fiddled with the front suspension quite a bit to get the big improvement from earlier years.
I don't think 150mm can do any harm, but if you find it a bit harsh after reassembling and taking it for a ride, lowering the oil level a cm at a time (on the bike :P) may be a good place to start. I have a small hand transfer pump I got from the local auto-accessories store - makes that very easy.
I actually wrote the springs in/caps off/fully extended oil level on the triple clamp to make this easier for myself next oil change - and yes, it was a lot less than the one with the springs out and the front collapsed.