imho (just ho) there's a lot of bollocks talked about longer forks messing things up on FS bikes.. it'll feel very slightly different, whether better or worse is up to you, just as likely you'll prefer it. then make adjustments with stem, bar, sag, angle-headset, tyres, bushes, spacers in fork etc. i would do the spacer thing last though as extra travel is nice and it's easier to change everything else (except headset, that is a pain).
(i just made a much more extreme change than +20mm and love it).
remember the particular angle you get any moment is largely a result of suspension travel and ride position, the terrain etc, not what it measures stood flat anyway.
edit: exception would be if you really don't want to change your current steering feel at all, then maybe just space when you get it! even then you're likely to get different axle to crown or trail or something, plus fork will feel different!
I would not use offset rear bushes to raise the rear end to compensate, not likely to work imho and raises your BB, see threads about this. use an angle headset if you need to, they work.