Assuming that is true with the APU only on the MB is it still true with an APU + dedicated graphics card?
Yes, it is even more true if you aren't using the GPU part of the APU. The only
advantage an AMD APU has over a similarly priced Intel CPU is the GPU part. If you don't use the built in GPU then it comes down to just CPU power and power consumption and Intel beats them soundly on both.
An A10-7850K is in the price range of some i5s, which completely destroy it in CPU power. Move down to the low end Pentium/Celeron CPUs (at least the Haswell based ones) and the similarly priced APUs aren't even in the same ballpark CPU wise, plus they lose a lot of their GPU advantage as well.
1. GPU still enabled because hybrid graphics are effective at increasing FPS enough for the user to care.
2. GPU disabled because hybrid graphics are a waste of electricity compared to the dedicated GPU.
All the benchmarks I've seen show #2 to be the case; hybrid graphics don't help enough. For example the best GPU you can use in hybrid mode right now is an R7 250. A Pentium or i3 + R7 250 will give better performance in almost all games than a A10-7850K + R7 250 in hybrid mode and be cheaper.
I may be the rare bird but I have no interest in buying Intel products and will continue to buy alternative CPUs. If AMD can't do it in x86/x64 the market will push towards ARM.
That is your choice of course, but AMD has lost even their price/performance advantage over Intel in most cases. I bought AMD exclusively, both personally and professionally when it was the better deal. Ever since the Core 2 that has not been the case. Professionally I cannot recommend AMD CPUs anymore as I would likely get fired. In the server space the performance gap is now so great that the higher cost of Intel CPUs is more than offset by the reduced number needed VS AMDs.
So look at the list of processors AMD makes the last time they introduced a new FX CPU at 95W or below was December 2012. The most interesting one on that list for me is
FX-6300 C0 6/3 3.5 GHz 3.8 GHz 4.1 GHz 95 W 2012-10-23
is the A10-7850K the fastest APU they have offered yet?
A10-7850K ?? 4 3.7 GHz N/A 4.0 GHz 95 W 2014-01-14
assuming a game plays nice with 2 cores but doesn't get any gain for the 3rd or higher core (which is common in gaming) and you are using the same discrete graphics in a PCIe x16 slot are these two CPUs roughly on par for gaming or is the difference in core design enough to make the FX CPU still better? If so how much faster does the APU have to be to offset the design difference vs the older FX CPU?
The A10-7850K actually has the "better" core design over the FX-6300 so is slightly faster clock-for clock. I say "better" because it is still a pretty bad design, just refined a bit more. AMD really has no excuse for the Bulldozer/Piledriver/Steamroller core design. They already watched Intel make the same mistake with the P4/Netburst, beat Intel at the time because of it, then went and made the same mistake. Lying about what counts as a core doesn't help any either.
If you must stay with AMD then the more interesting chip for you would be the A10-6800K. It has the same core design as your FX-6300, but a higher clock so it would be faster at single/dual thread tasks. It has a weaker GPU than the A10-7850K, but you have already said you would not be using it.