so, the question is, sure a mini spool is good to drag, but what about autocross or roadrace?? A moderately powered RWD car? Say a second or third gen camaro?? What would be faster, a gentle foot and a one legger, or a welded or spooled rear end and a slightly heavier foot??? Is a spool gonna be better than a grabby posi-traction set up??
i would say a welded diff or a spool would be pretty terrible to autocross or road race, and an open diff and a gentle foot would probably be faster.
Remember, the welded diff is hurting your turning unless and until you break traction on both rear tires, and at that point if you are in a turn you are probably not taking the fastest way through.
A spool will DEFINITELY not be as good as good limited slip of any kind.
For a lot of the popular RWD domestic platforms you can get selectable lockers which give you both worlds, but they are not cheap. 
I do think the more load you put on a welded diff the more likely it is to break, which is part of why i think ill be ok.. ill be welding a diff that only has 800 lbs over it and is turning 14" street tires. 
What is your budget for a LSD? You can find the s4 n/a clutch type out of an autotragic GXL for as low as $50 sometimes. You just need to swap the ring and pinion from the 3.9 to the 4.10.
YaNi wrote: What is your budget for a LSD? You can find the s4 n/a clutch type out of an autotragic GXL for as low as $50 sometimes. You just need to swap the ring and pinion from the 3.9 to the 4.10.
But they are usually dead as dead can be. The 3.9 is no problem though. 
I had a mini spool in my Pinto because 8" (not 8.8") LSD parts are stupid money. That was a drag car that was about 2500 with me in it. It lived just fine as long as I had it, and it's track duties saw 5700 RPM launches and 1.6X 60' times. And I daily drove it for probably a year, too. Scary in the rain!
I'd think that a welded diff would be under more stress in turns, not going straight. The spider gears only move when there's a difference in speed between the axles, so if they're going straight, the load is shared between them, if I'm not mistaken. And I very well may be...
Either way, it 'works' in a RWD car, but it also makes the car push/plow the front wheels big time when turning because it just wants to go straight. I had big n' little Weld Draglites on my Pinto, and between the spool and the short wheelbase, those skinnies took a beating. That car was light, but if it wasn't pointed DIRECTLY straight, you could not push it.
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