I personally do not know how fast Linux is on 3.15 or lower.
I haven't had the chance to see it on an older firmware.
I do know that it is rather slow and pretty much for "development purposes only" on the CFW's.
Depending on what you actually want to use it for, I seriously would consider leaving it on 3.15.
If you are primarily wanting to use it for backing up games............sell it to someone who mainly wants a PS3 for Linux and buy one with newer firmware (prior to 3.56)
If you are primarily wanting to use it for Linux.........leave it "as is" and have some fun with it.
I know that "in the states" the 3.15 machines sell for a good price, that is why I said what I did.
Perspex (one of the other well know posters here) will say that in India you can find 3.15 machines all over the place.
I don't know how it is in Aussie Land.
If PS3's with older firmware can be had pretty cheap, just upgrade to the 3.55 OFW and slap a good CFW in it.
It is not often that I am going to disagree with MadnessImport (he does have valid points on this), but I do have to (somewhat) disagree on this.
"Bricking" your system is not really a big deal if you follow one of the very good tutorials found here.
Unless you do something really stupid, you have a very small chance of turning it into a brick.
As far as a tutorial on BootOS.........do a search here and look in the Linux section of the forums.
I am way to tired right now to dig up the info for you. If you want to, shoot me a PM to remind me and I will try and find some good info in about 20 hours. I have a laptop with some links saved, but it is at my shop at the moment. Will try and remember to bring it home when I finish work.
It really comes down to what you are wanting to do with the PS3.......Linux or other stuff.
I would much rather go to my grave not needing my gun, than go there wishing I had it.