Don't bother with the DA chain. The DA/Ultegra/105 chains are all exactly the same minus some surface treatment, they are all within 1 gram of one another. The Ultegra cassette is believed to be much more durable than the DA one. Aluminum cage bolts are lighter and cheaper than Ti, and plenty strong enough to hold a bottle cage. A Selle Italia SLR saddle will save you 70g and probably be more comfortable than the Bontrager, and look nicer too. A Ritchey WCS 4-Axis stem will save about 15g or more with Ti bolts. Thomson Masterpiece seat post will save another 10g and be better made and again prettier than the Bontrager.

Personally, if the bike is already built up and you are going to spend the money to upgrade right away (I wouldn't) then do it all at once, you will be able to resell the parts for slightly more money if they are pull-offs and never were ridden, than if you ride the bike and replace as you have money or desire to. I would probably ride it until parts need replacing then upgrade at that time. Seems to make more economic sense.

But it's your money, it's your bike, do with it what you want.