Both assumptions are false. Hyperthreading doesn't lower game performance. Intel's architecture was specifically designed so that two threads don't get put onto one core unless all the other cores are filled up. Hyperthread also has no effect on the processor's temperatures. Since there is no physical cores, there is no extra heat generated.
If you want proof look at the i5's and the i7's TDP, both are the same. The ONLY reason to disable hyperthreading would be to test clock-for-clock performance against another CPU. I'm guessing you won't be doing this so leave hyperthreading on.
If, after ALL of this you still want to disable it, there is a setting in your BIOS that you can use.
I own the i7-3770K.
I can also confirm that there appear to be NO games that hyper-threading is causing any issues with.
Spider-Man Web of Shadows required HT to be disabled or it was unplayable, however on my i7-3770K it works great with HT still enabled (no idea why).
*I do have Windows 8 though so it's possible Spider-Man WOS may still need HT disabled.
The difference is next to NOTHING.
Leave HT on at all times. Most or all past issues with HT appear to be resolved.
Hyper threading doesn't effect games also it doesn't effect temperatures so not sire where yoi got those facts from. Also hyper threading increases basic performance by about 20% Keep HT on , if you don't want it should of got an i5
Intel's architecture was specifically designed so that two threads don't get put onto one core unless all the other cores are filled up. Hyperthread also has no effect on the processor's temperatures. Since there is no physical cores, there is no extra heat generated.
If you want proof look at the i5's and the i7's TDP, both are the same.
Intel's architecture has absolutely no control over what threads get scheduled to which hardware thread, this is entirely at the OS' sole discretion.
Hyperthreading DOES have an effect on heat output. The throughput gain from HT comes from the CPU's execution scheduler multiplexing instructions from two threads to maximize the number of active execution ports and execution units on every clock tick. With a single thread, the 5-wide output ports average only 3.5 instructions per clock while with HT, it should be able to achieve around 4.2 based on the a typical 20% worst-case gain. More active execution units per clock = more logic and DFFs switching = more power = more heat.
As for why i7-3770 has the same TDP as i5-3570k, do note that the i5-3570k is nowhere near 77W even under full-load (my i5-3470 peaks around 30W) and that 77W includes the IGP most of us on THG aren't using, so there is plenty of room under the 77W TDP to accommodate the ~10% extra core power draw (most of the instruction scheduling logic operates at full power regardless of whether HT is enabled or not) that might be caused by HT.