Iphone 3gs flash ipad baseband 6.15 then downgrade possible on newer serial #?

I'm not entirely sure if this will work as some say it'll brick, while few others have claimed it'll work. Can anyone verify or confirm this for sure? Basically will these methods work?

Iphone 3GS with newer serial # manufactured "after" 134 (2011, week 34).
1) Flash ipad baseband 6.15
2) Then downgrade baseband using cydia to baseband 5.13
3) If above step doesn't work, downgrade to custom firmware 4.1, then use cydia to downgrade baseband to 5.13.

NOPE,
you need the matching SHSH Blobs for this to work [which are no longer available or being signed anymore], you can't use an IPAD SHSH for the Iphone 3Gs [& Visa Versa]

the Ipad SHSH only works for ipad
& the iphons 3gs shsh only fors for iphones,

understand?
don't do it or youll brick

also see:
http://www.iphonehacks.com/jailbreak_iphone