180 is great for a 500.  250's around 220.

When you tested, did you hold the throttle wide open?

We'll need a lot more info for figuring your jetting but i can tell you that 172 in that small carb on a 250 seems really really massive.  I ran the 39pwk bored to 40.5 on a slightly ported 500 and i think I ran mid to upper 170 main. 180 main.

Might buy a few genuine Keihin jets and start stepping down the main until you get your powerband back.  Maybe do a plug chops at full throttle to be sure you dont get too lean. 

Do you know if your motor is air tight?  have you done a leakdown test?

***Edit:  I remembered i ran a 185ish main.  So maybe 172 isnt far off?  Either way, do some plug chops and see if you're lean/just right/rich on the main and go from there.