Point 3 depends on the workloads. Most SMT [1] implementations replicate integer functional units - otherwise the threads would stall on basic things like computing addresses - but they don't replicate floating point units. So if you have lots of floating point heavy work, then you're limited by the number of cores, not the total number of SMT contexts provided by all of those cores.

So it's not that SMT pipelines are slower, it's just that they share resources with the other SMT pipelines.

[1] Simultaneous multithreading (SMT), http://en.wikipedia.org/wiki/Simultaneous_multithreading, is the generic name for what Intel calls hyperthreading.