Actually changing the arrow weight will not change the KE to any measurable degree.

KE is a function of the bow not of the arrow. If the arrow mass is reduced, the speed will increase and KE will remain effectively constant.

Another consideration with going to an 85gr head is that your arrow will act like a stiffer arrow, and may make your bow more critical to tuning issues.

IF you are looking to pick up speed, there are a number of things you can do to gain speed without changing your draw weight or arrow.

#1) Remove brass nock sets from your string. Removing a single brass nock at the nocking point can increase speed between 3-5 fps. In stead use a tied on nock. These weigh almost nothing and can be tied on in a few minutes.
#2) Get rid of cat whiskers, puff balls, etc....in lieu of something like a string leech. You could probably pick up about 2 fps there
#3) DO THIS ONE ONLY IF YOU HAVE A GOOD QUALITY AFTERMARKET STRING.....Get rid of your peep sight with an aligner tube. This setup will rob you of 3-5 fps. Something like a Specialty Archery Super Ball peep or a G5 Meta Peep should fit the bill nicely.
#4) You will need access to a chronograph for this one.......Add a set of accelerator balls to the ends of your string near the cams. These have been shown to add up to 7 fps on some bows, however you need a chronograph to find the "sweet spot". Many bows will also have a "dead spot" where the accelerator balls will actually cause a speed loss.