From what I've read, the series 8 has different turbos and larger ducting for the oil coolers, radiator and intercooler. Are there any other major differences between them mechanically that are going to significantly increase reliability?

 

I think you've covered the main changes. The ecu was also upgraded to work faster and adapt to fueling changes better, plus the exhaust walls were thinned out a little to increase the bore of the exhaust. I also think I read somewhere, that some of the hoses/solenoids etc in the "ratsnest" were also changed, to improve both reliability and smoothness (i.e. help reduce the 3k and 5k hesitiations). Size of the rad was increased (as well as the opening already mentioned), and I think the angle at which it sat, to. Other than those, can't think of anything else.

 

I'm not sure which series motors it went into effect on, but the rat's nest was done away in favor of the "black box" vacuum box, there was better lubrication for the apex seals, different oil metering pump, 16 bit ecu and different harness. . . . . thats basically it IIRC. Well, the major stuff anyhow. Someone please correct me if i'm wrong.