It's not about power or straight line speed. The 86 has always focused on cornering ability achieved through balance and being lightweight. NA engine and manual are crucial aspects, too. Watch this for an idea:
These cars are tuned, but the video still shows the strengths of the GR86.
Keiichi Tsuchiya (the Drift King) had a
positive impression of the outgoing 86, although it's definitely slow on a track. These current or former professional drivers have tested pretty much every sports car under the sun. They basically agree that more power is good on large, open circuits, but on narrow, winding roads, it's all about cornering and weight. Their personal cars are along the lines of AE86 and MK4 Supra, although they could afford things like the M8C or AMG GT R which they've tested. A 340hp S2000 is faster (by 0.8s, ~25s lap time) on this course than the 577hp AMG GT R.
This is a good demonstration of the impact weight has between two cars with similar power:
I do agree that it could use significantly more power, but that's ultimately not the focus of the platform (identical to BRZ). You can always add power yourself. We all know these engines can handle way more than stock power with original internals.