Non-reference designs can have few or many modifications from the reference designs. The Asus TOP series. MSI Hawk / Lightning and EVGA Classified series are examples of cards which historically have many changes such as improved PCB's, better coolers, extra phase VRM's, VRM heatsinks etc.
Other models such as the EVGA SC which, again historically, have very little modifications. Here's how that shakes out with the 670http://www.guru3d.com/article/evga-geforce-gtx-670-sc-review/23
EVGA SC Boost Clock is 1046 outta the box....stable at 1200 Mhz
EVGA SC gets 3DMark Score of 8691 outta the box and 9443 OC'd
Idle temp is 29C / load temp is 78C
Noise is 37 dBA Idle / 43 dBA under loadhttp://www.guru3d.com/article/asus-geforce-gtx-670-directcu-ii-top-review/23
Asus TOP Boost Clock is 1137 outta the box .... stable at 1280 Mhz
Asus gets 3DMark Score of 9340 outta the box and 9839 OC'd
Idle temp is 30C / load temp is 72C
Noise is 37 dBA Idle / 38 dBA under load
That makes the Asus 7% faster outta the box than the EVGA SC, 6C cooler and almost 1/4 as loud under load (every 3 dBA is a doubling of sound level).
If set on EVGA, and planning to OC, I'd get the Classified version. The SC Signature and SC Signature + just don't have what the Classified offers.http://www.maximumpc.com/article/news/evga_announces_two_signature_series_gtx_680%E2%80%99shttp://www.guru3d.com/article/evga-geforce-gtx-680-classified-with-evbot-review/
EVGA Classified 680 - 10,924 in 3DMark 11