Higher VCO frequency

The max spec for SII PLLs is 1040 MHz even in fastest speed grade. You can't get higher than 1040 MHz VCO unless you lie to the tool about your input frequency. E.g., you set for 100 MHz input with 200 MHz output and 125 ps phase step. This will set VCO = 1000 MHz to achieve the 125 ps, then divide that by 5 to get the 200 MHz output. However, if you set the input frequency to higher than 100 MHz, you will be running the VCO higher than it is specified for.

With your frequency case, 266 MHz output requires a 800 MHz or a 1066 MHz VCO. The latter is what you want but its illegal / out of specification but only by 26 MHz. You can fool the tool and enter that you have 250 MHz output but then run it at 266 MHz on the board....but you are technically running out of spec.