Not a direct comparison, but I'll provide some input anyway.
We purchased our 2002 Islander 270 new with a single 250HP OX66. Boat is rated at 500HP (more than the 268), and the 50% HP was marginal. We could cruise comfortably at 22-25kts, and top speed was just over 30knts, but the engine was working hard and fuel efficiency was poor. The other configuration in 2002 was twin F225s, but we purchased the boat show boat already rigged.
We repowered in late 2005 to twin F150s. We gained five knots on our 'comfortable' cruise speed, with 35-40% better fuel efficiency even at the higher speed. And top speed increased to just over 37kts. We now have over 1115hrs on the twins, with many trips 75nm offshore to the Norfolk Canyon area. With five persons, full fishing gear, full fuel and water, 250# of ice, etc, we generally get about 1.5nm/gal.
The 268 has a different configuration aft, with the bottom running surface ending further forward. This affects the HP rating, and the overall ride and performance a bit.
IMHO, a good pair of 150HP Yamaha 2-strokes is a good match for the boat. I would look into the fuel use if those are carbed engines, and determine if the boat's range with the burnable fuel carried will meet your needs. Our boat has a single 150gal tank, and the 1995 model 268 had a 134gal main tank - with a 71gal auxiliary tank as an option.
Contact me if you have any specific questions.
Brian