We purchased our 2002 Islander 270 new in March 2002 with a single 250HP OX66. We put 465 hours on it, so I can fairly comfortable assessing that arrangement.
That boat is rated at 500HP, so with 50% power it was adequate. Cruise was 22-24kts, and WOT was 30.5kts. Both with full load, top, curtains, etc. Engine was very powerful, very reliable, and very thirsty. We usually got 1.2-1.3nm/gal, but it could drop to 1.0nm/gal quickly in a chop, or if you pushed the throttle up past best cruise speed. The beast also loved that 2S Yamalube...
We repowered in Nov 2005 to twin F150s, and now have over 600hrs on them. So again, I can speak from experience.
The 270 with F150s is a different boat than with the single OX66. Cruise is 25-28kts, with WOT of 37.5kts. Cruise economy is about 1.6-1.7nm/gal, at the higher cruise speed. WOT is still 1.2nm/gal, but we rarely run that way for long.
While you may compare the 232 and 268/270, the Sailfish is a different animal. Has to be twin power, and the 200HPDIs were great (same on the 265).
If you are looking at older boats and engines, make sure you know if the engines are carbed, EFI, or DFI. It makes a BIG difference on fuel economy. And, while that certainly affects your $$$/nm, it also affects your safe operating range with the installed fuel. That was the primary reason we repowered - to be "canyon-capable" with the 150gal tank.
So, you also need to look at the installed tankage on the boat. I don't know about the older Gulfstream, but the 268 Islander had 136 gallon main tank.
Best wishes in your search.
Brian