Just a couple of precisions.
GT5 has already taxed the PS3 to it's limits,
kaz already stated frame rate dropping on the PS3, there is no way to "work on the PS3" you can do firmware updates, but you can't fix the 256mb of Vram the RSX has, Sure it's fast, but speed can only do so much.
Uncharted 1 taxed the PS3 to its limits, they released a second one. GT3 taxed the PS2 to its limit, they released GT4, why wouldn't they release an "easy" second game, it's "almost" the same program. It is a better investement to develop a 2-3 year old GT6 for the PS3 than thinking about a console that doesn't exist. They have to work on GT6 either way, they will release what they have by then on PS3, believe me.
Anyways back on topic of hardware, the way how Video Rendering devices work are, when you have a device that is giving data to a TV, a lot of things are at hand, resolution, color, dpi, contrast. the higher these figures, the harder it's going to tax the hardware.
DPI has nothing to do with the source signal, it's just the pixels' size on the display device. Contrasts are irrelevant as they aren't part of the signal, and contrasts are higher in SD CRTs than your best HD Plasma or LED, full black and full white have been around a long time too, contrast levels will depend on the type and quality of your display. Color depth is irrelevant too, computers have been using truecolor, wich is beyond our perception, since the 90's and the PS3's hardware doesn't have the power to use alpha blending and aliasing that would justify deep color either. This is all very well known territory, just like fuel injection, it might look fancy but it's really not.
We have had a HUGE jump in what LCD/LED TV's can do since the PS3 came out, 3d wasn't around, LED TV's weren't around, TV's in 1080p (Companies started offering 1080p in 2005), and pure 1080p games JUST started coming out on the PS3. When the PS3 first came out 1080p was just a new thing, now all these TV's are getting MUCH better quality, and the games are starting to use even greater quality, put the 2 together and you get a very taxed machine.
Sony knew about 1080P, 1080i existed and progressive was on its way. A 1080P source signal is the same on the first 1080P LCD as the most recent LED, it's the same as 720P. 3D is new, but if you don't play in 3D the 2D shouldn't be affected, technically, in practice it might, but don't blame the TV nor the PS3, blame the game developer.
Just the Cell CPU was a 4 year, 400 million dollar project for the PS3. So they'll take just as long developing a new one for the PS4. 4-5 years from now (like i said) It just adds up to me is all.
I don't understand why you think Sony would put as much R&D into the Cell 2 as the original one? They already designed it, there probably be improvements, but nothing major as 4 years, with 4 years you'd get something completly different and the studios will have to re-learn most of its technology. The PS4 in itself might take 4 years to develop, but the processor alone, doubtful, especially when it already has the power for the PS4, all it needs is a faster RSX and twice the ram, though I see Sony going for a bigger, much bigger, Cell and total removal of a second graphic dedicated chip, its cost effective and way more powerfull, since everything happens in one die. Third-parties won't like it, but they never sold systems, unless they go exclusive.
, and they are also killing 
