The i9 will get you more cores.
If you run workloads that can make use of more cores then yes, maybe.
But the whole macbook lineup at the moment, hardware wise, is a joke. Well, the 12" is reasonably good at what it is aimed at, assuming the keyboard issues are fixed (will give apple benefit of the doubt for comparison purposes, but i wouldn't buy one yet). But that USB-C port should have thunderbolt.
That said, all machines throttle to some degree these days.
They make use of thermal headroom if any to temporarily raise clocks. Sometimes that headroom is only available for short bursts.
For most desktop/notebook users their workload is quite bursty, so this works well.
If your processor is running at its base clock or faster, it is NOT throttling. The base clock is the only speed you're supposed to EXPECT from a CPU. That's the clock it is designed to be able to sustain in the rated TDP. If you have great cooling (and stuffing an i9 in a modern macbook is NOT an example of that) then you can maybe sustain the turbo clock. But the CPU itself is only designed to be able to sustain the BASE clock, at the heat dissipation it is rated for.
TLDR: if you actually want decent performance in 2018, you shouldn't be looking at Macbooks.