Ok... Suisin... honyaki or not? If you think of honyaki in the traditional sense of mizu-honyaki (water quenched differentially tempered) then it is not. If all honyaki means to you is made of a single piece of the same metal then it is. I take the more traditional view on this and refer to knives like the suisin as mono-steel knives.
There are indeed a number of other makers that make super thin wa-gyutos. The shape of many of them is nearly identical. What differs is the type of steel, heat treat, and handle. Suisin is not necessarily always the thinnest, but they are all so freaking close it doesnt matter.
To answer your original questions, it can not get sharper than the hiro, and it will not hold its edge as long. It does sharpen really easily though and i have used it in pro kitchen environments with no real probelms. This is how it is better than the hiro in my mind: Its so freaking thin it doesnt matter if it can get "sharper" than the hiro... it feels sharper than the hiro because it doest wedge in anything ever. The edge is less brittle than the AS in the hiro, so it wont chip as much.
I've sharpened mine to a stupid thin edge of less than 10 degrees... maybe closer to 5 (or even less... i dont measure). I know that i loose some edge longevity with this, but i love how freaking sharp is can get. As will technically be able to get sharper, but the suisin feels sharper. As could hold an edge like that, but it would be super brittle.
If you want a super thin gyuto, pick either the suisin or the tadatsuna (depending on what handle you like). They are identical for all intensive purposes (same shape, geometry, and HRC). If you want a knife with the same shape and geometry, but slightly softer steel (which makes it slightly more durable- as in less brittle and better with poly boards- in the kitchen and a little easier to sharpen, although you may have to sharpen a little more frequently) there are a number of options form various makers. Here's the short list: Takayuki Grand Cheff Wa, Ashi Hamono, Mizuno Tanrenjo, and Konosuke (I'm pretty sure i'm missing a few).
For the record, i love my suisin... heres a picture of my baby: