You will more than likely have to go with a 15.6" screen at minimum to get the features you want. There just aren't any smaller options aside from a MacBook Pro with them avaible.
You are not going to like it but I'm tossing the MSI GE60
into the fray. Its designed as a gaming laptop so its 15.6", about 1.5" thick, comes in at about 5.3lbs, and battery life isn't great.
Easy access to internals
Official support for 16GB of ram
Full fledged mobile i7 quad-core
open mSATA slot for SSD + 2.5" HDD bay for storage + an optical drive
Starts around $1000
Other options in the 15.6" range:
- Lenovo's gaming laptop. Pretty much the same specs as the MSI however max supported ram is 8GB(though some people have had success with 16GB) and it weighs more at about 6lbs.
- Best professional 15.6" option. Multiple quad-core i7 options(up to extreme editions), 1080P screen w/ 95% RBG Gamut , up to 32GB of ram(4-dimms), mSATA slot + 2.5" drive slot + optical drive or additional HDD w/ bay adapter, Kepler based NV Quadro options, huge 9-cell battery, Windows 7 or 8. Biggest con is that it is heavy, around 6lbs and bulky even by 15.6" standards because of the battery. Another con is price: starts $1400 in base config with quad core, 320GB HDD, 8GB ram, and a 1080P screen. Review
- thin ultrabook like 15.6", only 4lbs with 8-cell battery, 1080P IPS screen, up to 12GB of ram, base model has dual 128GB SSDs. Limited to ULV dual-core i7, limited availability, and expensive: starting around $2k.