(Never having owned a kobra, but having a favorite 18" Sher WWII and beautiful 20" Bura Sirupati...)
I think the Kobra and Sirupati are pretty comparable. Lots depends on the thickness and weight balance. Some Kobras are beefier than some Sirus. IMHO, even the thinner Kobras and Sirus are fine for chopping up to about 2" diameter wood, provided it's not extremely dense, knotty, or unusually hard. Blade speed for both is going to be high, making them good for machette-type applications (springy brush, vines, etc.) Larger soft or green wood is still easily chopped. Lots depends on your technique and having a good edge on the blade. With practice, a fairly light Kobra should be able to perform well as a chopper.
The extra belly and consequent forward weight of the WWII makes it chop better than a Kobra or Siru of the same approximate dimensions. This is most notable in harder woods and thicker branches. All 3 will go through small branches in one or two chops, but bigger stuff takes more effort.
The WWII will quickly give you enough wood for a good sized fire, but if you need even more chopping power, the next step up is and 18" AK - then bigger AK's and the Ganga Ram Special.
Finally, the Sher WWII in the top picture is beautiful, and a twin to mine. Notice that the area between the Handle Ring and tip of the Sword of Shiva where the blade turns down is slightly curved, while the same area in the Sirua and Kobra is almost straight. Maybe it helps in chopping, maybe not, but it sure is sexy.
