Certain times,like when someone wants the same look to the gun, I go to the rear sight first and take it down some and try it...a little at a time so you don't end up shooting low. best to take off the rear as you are shooting. I use a diamond file easily aquired at the hardware store. You don't want to take too much off the rear sight because the rear,breech end, of the barrel can start showing in the rear sight. Some rear barrel in the bottom of the notch isn't bad but too much and you can't get the front sight in the rear sight.
So the gun doesn't end up with too much off the rear sight and too tall a front sight I'd do both...the hammer sight and the front sight. The front sights can get difficult to get out and may need filed off some and then drilled out(a bottoming end mill is much better than a drill bit to take the brass out of the hole) and replaced with a higher one pressed and loctited in . There's always some quirp to everything. Like,"easier said than done". That's why gunsmiths get top dollor for their work.
Some take off the rear sight at the wrong angle and that makes for more that has to come off. Denster has it right. The top of the rear sight should be paralel to the barels centerline. Less comes off that way and it doesn't look noticable really. Just put the gun barrel in a padded vise and pull the hammer back and cover everything with a rag so filings don't go down into the gun and....tape the top of the barrel with two or three layers of masking tap....so the file can't get to the barrel. Eye ball the sight from the rear to get the top on the right plane and not canted and tell when the sight is getting to the point the barrel shows in the notch. Remove the tape once in awhile to check for "barrel" beginning to show in the rear sight. Deepen the rear sight notch some usually with a triangulat file OR a round file. I like the rounded bottom to a rear sight since it lets more light show on each side of the front when aiming.
Anyway take the rear sight down while shooting the gun so you know when to stop...unless you have some "Kitchen Table Gunsmithing" under your belt and can tell how far to go. Then if the rear sight goes all the way and the gun still shoots high you go to the front or live with a six o:clock hold.
I'd shoot the gun more to make sure it's broken in before changing the sights.
Check the crown first since that may be off and make the gun shoot higher.
Sometimes it just takes a,"firm" grip on the gun and it'll shoot lower from the weight of the arm. Make sure you ain't "limp wristing" the gun making/letting it shoot high. The gun mentioned seems to shoot "pretty" high" and makes me suspect some of the "high" may be from the grip the shooter has on the gun.
Maybe the rear sight just needs opened up so the shooter can get the front sight down in the rear more.
I'd check the looseness of the arbor in the barrels hole first off too. If the arbor is a loose fit the barrel will cant down at the rear and up at the front. A shim may be needed in the hole with the arbor....on the top of the arbor and inserted with the arbor into the barrels hole. Piettas usually aren;t loose arbors in the barrels hole but it can happen.
In the long run it would be easier to leave the sights as they are and get used tio the "shooting high" and maybe......learn why the gun does shoot extremely high. May be something other than the sights. The arbor barrel fit loose is a biggie. The front muzzle crown is another. The "firm" grip on the gun can help it shoot lower. The barrel being "quirpy" may be the cause of the gun shooting so high. Maybe a "loose" spot at the muzzle or somewhere inside. The rufkling grooves being "unequal" depths may cause it to shoot high. Look for a tight spot inside with a lead slugg "bumped up to be tight in the barrel. Measure a slug from the front about "not vaery far in". Do the same from the breech end. Then tap a slug clean thru and measure and see if there's a tight spot inside that needs lapped out. Maybe the muzzle end is more diameter than the rest of the barrel too. That can make a gun shoot badly. A smaller diameter at the rear and a larger diameter at the muzzle end is "bad". There are times a barrel has to put in the parts bin to save for a "snubbie" and a new barrel bought. New barrel bought since all the retailers won't take back a gun because it shoots high because it's got a bad barrel. If the gun is "unshot" they will(well some maybe). I just ordered three guns from Cabelas and sent them all back because of unequal rifling grooves(makes a gun shoot bad). They wouldn't send a forth(same model on sale)out to me. It was said they didn't have in stock what I needed. I had called to complain and I guess they figured I'm too picky. I am picky. I won't buy a cap&baller rev that has unequal riflings depth or...the muzzle end shows the barrel was drilled with "run out" and the bore isn't concentric with the outside. That means the hole was drilled off center and then rifled and the gun will shoot off. If the bore cants down a little so the gun shoots lower I may keep that.
