Well you asked several questions, so I'll try my best to answer each one individually:

Q-->"If you have the lugs lapped on a bolt action, will the headspace change enough to have the barrel set back??"

A-->It depends. All you can do is lap the lugs sufficiently then check with go/no go gauges. If it removed enough material to all a the no go guage to close then Yes you will have to set the barrel back.

Q-->If not, will lapping the lugs have a noticeable effect on accuracy? Now shooting 1 1/2" groups at 100yds. This is acceptable but would like to tighten up by 50%."

A--> It depends on what you consider noticeable. 50%? NO! Itsmore ainstrument in an orchestra you have to have all the other instruments in tune as well.

"It is a synthetic stock so I hope thats not an issue. I did bed the recoil lug."

A---> Bedding the lug is insufficient in my mind. I like to pillar bed to relieveall stress. Then completelyglass the action and the first inch of the barrel. Free floating the rest of the barrel.

Also, as a side note the best money youcan spend for accuracy is a custom barrel. Even on an "untrued" action theaccuracy improvements are unbelievable. I've personally seen a gun that shot 1.5 moa go to .75 moa with amatch grade barrel. Now after that we trued the action, lapped the lugs, bedded, freefloated, addedJewel trigger.... Any guess what it shot then...... .5 Moa. Make your own conclussions.

Its kind of like the race car engine game. Getting 500 hp isn't too difficult or expensive. Going up from there gets harder and more expensive for each HP. Getting close to .5 moa is relatively easy and not terribly expensive.Going down from theregets increasingly more expensive and difficult.