I love my GT8 and see no reason to ever upgrade to the GT10. I use it much like you, just for 4 cable method with my 5150 to get a solid clean sound and even my old GT5 excelled at that. 
The GT10 looks gay compared to the GT8 too  
		 
I know, right?  GT-8 looks absolute beast.  I think the GT-10 maybe exists to just milk the market.  Nicer screen, but bullshit to that if the GT-8 does the same shit.  
Read some stuff on cables the other day, some dude recommending cables with twisted-pair conductors, particularly for the cables coming in and out of the amp's effects loop to keep the signal clean.  I need new cables anyway, so if I go this route I'm looking at Planet Waves Custom series.  They're decent priced, too.
I wanna use my SD-1 with my 6505 preamp always, won't ever need to turn that off, so I guess I can use 4 cable + 1 patch, I figure I can go from this:
- Guitar to GT input. 
- GT send to amp guitar input. 
- Amp loop send to GT return. 
- GT output to amp loop return. 
to this:
- Guitar to GT input. 
- GT send to Boss SD-1 input.
- Boss SD-1 output to amp guitar input. 
- Amp loop send to GT return. 
- GT output to amp loop return.
If my theory's right then when using the pre-amp contained in the GT loop, I'll always be going through the SD-1 first, with the SD getting bypassed whenever the 6505 preamp is.  
I guess with no pre-amp the 6505 is silent?