Honda 5 hp outboard throttle?

-hi, im just getting my 07 4 stroke honda 5hp outboard ready for the season. i bought it late last year and am curious of a couple of things. im loosing about 25% of my throttle which seems to be down to a stretched cable? i think... also ive noticed my engine will not rev very high at all. when i take the cover off and open the revs by throttle it (butterfly on top of carb) opens about 7-10mm. but when i pull the choke it opens twice that and opening by thumb openes it about 3 times as much. is this as it should be? also. at the throttle bar (between the cable and the carb) i noticed a semi circle between 2 pins that stop it from getting too high and too low in revs. would it be a bad idea to take a nick of the end of this semi circle to allow it to open more? anybody have any ideas? would a new throttle cable fix some of my problems? but could i open it more by use of mister hack saw? its my first engine but same honda engine on youtube seems a lot faster and higher revved. also what oil should i use in it. thanks in advance for any reply. my engine seems great. no oil problems, pressure, overheating or rattles. alsosex