you will lose more than just top end.
lower octane fuel will hurt your gas mileage in a car that is made to run premium and it will also pull from your power. yes you can run regular, but you are better off running at least 89 or higher or the manufacturer's recommendation.
i run 93 octane unless i cant find it then i will run 91 and ill add a bottle of STP octane boost. ive run 89 on one fillup but i only put 3 gallons in to get me to a place with higher octane gas. 87 has never touched my tank.
another thing to keep in mind is that a 3 pt octane booster will not bump 89 up to 92 octane. Under the North American octane system AKI (Anti-Knock Index) pump fuel is graded as (RON+MON)/2. Meaning if a fuel has a RON (Research Octane Number) of 96, and a MON ( Motor Octane Number) of 90 its AKI would be 93.
basically, you add the octane rating to the MON. so for our 89 octane the MON is roughly 90 and the MON is 88. add 3 to 88= 91
so in effect you are getting 1.5 points even tho the bottle says 3 pts.
here's a good writeup with MON and RON testing of octane boosters