It's not like the ground is crawling with critters, yeah their out there but it's so rare to have issues.
2.5 pounds sounds heavy to me even with all of my strings I don't think the tarp bivy combo weighs that much.
I linked this at another thread
http://www.rayjardine.com/ray-way/Tarp-Kit/index.htm Ray has been around the block so to speak, he's big on open air sleep systems for breathability. I will say I prefer to sleep under a tarp instead of a tent. Under the stars, is the best of course, if it's clear.
No matter your intended finish time, trust me you don't want extra stuff. It's not easier to be slower, unless your a super fast guy just cruising. Most of the slower riders seem to be working just as hard or harder then the faster guys. Carry less have more fun, just enough for a good safety margin.
As to pads, again there's a huge range from nothing, to improvised car shades, to z rest foam. I went with the Pro Light back in 06 and still use it. There are newer lighter air pads now. Just make sure to lay out a ground sheet of some kind and be sure to check for thorns.