Heres my opinion on whats been discussed here. i truely beleive that the SP400 with auto LSD is not street friendly at all. Can it be done, of course, is it better for the track, Hell yes. but the Sp400 with 6sp LSD is the way to go if you love driving on the street with occasional racing.
With the 6sp LSD, i can cruise at 70mph @ 3K rpm's. With the auto rear you will see rpm's in the 4K range.
The 6sp LSD is hurting me at the track, because of the gearing difference, it is harder to get out of the hole, but i sacrafice that to be able to cruise on the hwy up to 180mph, get better gas milage, and honestly my auto rear end was not a LSD, which it was supposed to be.
If you do more racing then cruising around, get the 400 with auto rear and transbrake, you will have a blast. If you like to cruise around town, go on trips, ect... get the 400 with 6sp LSD and lockup, or if you have cash just get the overdrive unit.
Long cruising for hours definatly needs a lockup. When im cruising and i lock it up, within 5 minutes i notice my trans drops at least 20-30 degrees. In summer heat, cruising without a lockup, you will melt something. I should know, i did.
regardless of setup, get 2 tranny coolers, the biggest you can fit, and preferably the new B&M ones with the SPAL fan attached to iy. your trans will thank you.