It's 7.9 lbs, but it'll be _stable_ if it's anything like true to its specs.
An extra $40 gets you to:
http://www.bhphotovideo.com/c/product/483925-REG/Giottos_MT9371_MT_9371_Aluminum_Tripod_Legs.html
that sheds a pound and a half
I don't think that head will hold up over time- this one looks interesting for a knock-off:
http://www.bhphotovideo.com/c/product/498020-REG/Benro_450_622_KS_1_Ballhead_with_Quick.html
But you'll be replacing it inside a couple of years, at least the plates are Arca-Swiss compatible, so you won't lose anything there when you go to the next level. I've sunk at least $300-400 in cheap ballheads over the years, at first they're good, then they creep, then you're at the store handing out more money.
Here's the A-list of ballheads:
http://www.websiteoptimization.com/speed/tweak/ballhead/
I know it's expensive, but it's holding all your expensive gear, so getting a head that's not going to fail is good, getting a head that you're not going to have to re-buy is good- if you get serious use out of it, a $100-200 head is probably going to have to be replaced in a year or two- especially with heavy lenses. If you spend up-front, you'll have the same head for years- it's really cheaper in the long run. I've never had luck with the smaller Bogen QR plates, or the Hex ones, the tombstone ones are the only ones that I've had any luck with and those tend to break inside 2 years- the ones I used are discontinued and replaced with the 410PL/RC4 combination.
The Arca-Swiss style plates are nice in that you can get long ones for the tripod foot on the 300, and short ones for the body and still be assured that your lens/camera is fully supported. Like I said, you're going to end up with the good stuff anyway- it's just if you drop $150-500 on the in-between stuff before you end up there.
Definitely- you'll see when you start shooting that being able to go to 2.8 @ 300mm is going to give you what you want results-wise for sports, and you'll find that you're going to be wanting more reach for nature- but sports and nature photography are both expensive propositions done right and you can do well with a 300 if you're quiet/patient.