If you must mount it on the coffee table, I would try to get a coffee table with a lower shelf and mount it in that, then route the cables underneath the coffee table, under your couch, then along the baseboards. I hate seeing wires, and I like tripping on them even less. That said, you will see a phenomenal improvement in comparison. You are getting:
1280x720 = 921 600 pixels vs 1920x1080 = 2 073 600 pixels in the same screen size. What this means is a much clearer image (that, and you can sit closer before you begin to see pixelation, so it's more immersive). You may not like 3D, but the processing power needed to run the 3D also means a much cleaner 2D image. It is leaps and bounds above my old BenQ W5000 (a 1080p unit). It is the cleanest image I've ever seen. Amazingly, the 3D image is even more clean/clear than the 2D one. I'm not sure about black levels, but your H78 is a Dark Chip 3 and may trump the HD3300. The blacks are plenty fine for me, though.
About brightness, it is definitely plenty bright. Especially if you plan on having it in a coffee table in front of you. Good luck!
Video: Optoma HD3300, Panamorph UH480, Darbee Darblet, Samsung BD D6700 Blu Ray player, DIY Seymour Centerstage XD 138" Curved Screen
LCR - Tannoy DC12i, Sides - Tannoy IW63, Rears - Tannoy Saturn DC6ti Bookshelf, Ceilings - Tannoy CMS 603DC (TF), Tannoy CMS 601DC (TR), 3 Micro Marty/1 Full Marty DIY subs
Amplification: Emotiva XPA-3, Emotiva XPA-2, Outlaw Audio 7700, 2x Inuke 6000DSP
Receiver: Anthem MRX 720