Black locust makes the best fence post materials. I wish you were close by and I would go clear your place if you need to get more! We are still building our farm towards cattle and need to add another 15,000ft of fence to the 23,000 we already have built!
Bottom line for locust? Power! The smallest saw I would use would be 55cc. If you have a lot of it, something like a Husqvarna 359 or bigger, or a Stihl MS290 or larger.
I use my old 80cc Hobbie Homelite XL800,s XL903's and Super XL925's (Have maybe 150 bucks in each one of them) and they rip and tear through that hard hood like nothin.
I used locust from our hedge rows for a lot of the line posts...actually cut the larger ones from end to end (9ft long) into quarters to get the line posts...here's A picture of the Honey Locust getting Homelite XL903 Surgury..not the Black locust which I did the same thing with.
Also at work, we use Husqvarna 357XP's for the road side clearing of trees and brush. Locust is...the most common tree we have to clear. They are enough power to do the trick.