So if I had a slight tip strike that ended up taking about 1/4" off each tip of a 3 blade prop to get it fixed and rebalanced - Has that affected the performance enough to be a problem? thanks.

It might work better..?

How big is the prop?

APC props generally don't like being shortened or messed with, other than shaving the blades for experimental speed trials. This is one of your "tools" for tuning a piped engine.

Master Airscrew props can respond very well to modifying / shortening.
Same goes for most wood props I've ever tried.

If your shortened prop runs smoothly and has no hidden damage then give it a try.

Assuming it was a very gentle kiss that ground off the tip and not something that snapped the bits off so that it didn't affect the root area of the prop you should be OK. I'm far more worried about the structural integrity of the prop after stuff like this than the motor to prop efficiency. You should be as well.

You don't give any idea of the diameter difference between before and after so it's hard to say. But if it's something like a 1/2 inch from a 10 inch prop then the difference may not even be noticable. If it's a half inch off a 6 inch prop then forget it. Toss it and get a new one.

Should have mention it was a WWII style wood 20x10x3. The damage and repair has made a 19.5".

If this is the type of prop that is made of 3 seperate blades that is glued together with plates and pins, I would say don't trust it. The safety issue is just not worth the cost of a new prop.

He could use it on a scale RC lawn mower..?

Wouldn't try it on the lawnmower. Having the blade come apart that close to my feet is a scary thing. Ihad a mower drop a blade once and it missed my foot by 2 inches.

Seriously:Inspect the hub carefully. If there are no stress cracks, and the prop tracks true, trim the tips and balance the prop carefully. if you are really confident, bolt it to the motor and ground-test it in a safe location. Stay behind the plane and make sure that anyone around also is behind the plane. Run it through the entire range of engine speeds and make sure it does not hit a destructive resonance. Be ready to chop power instantly. When it survives the ground test-go fly.
Land carefully and Don't bounce it off the ground again :-)

The problem with doing a good ground test is that if it does throw a blade the nose of that airplane is going to literally explode. I saw a 3W Votec have a spinner fail once. The spinner lip delaminated ( fiberglass spinner ) which allowed the spinner to shift around 3/4" to one side. The resulting vibration immediatly pulled the engine off it's mounts and the pilot had a running DA 150 loose bouncing around in the cowl. I shutter to think about what would happen if an airplane threw a blade on the ground and the engine ejected onto the gorund. It's just not worth the risk!

I had a 15x? prop explode pulling a Goldberg Ultimate straight up and the counter-reaction ripped the entire firewall off in a split second. The throttle cable and some of the cowl kept the engine from completely taking it's own route back to Earth.

Right and from the prop size mentioned, I'm thinking it's attached to a G-62 or equivilent

You guys raise a good point on the ability of such engines to totally destroy the whole front of the model in a blink. Ideally the engine and prop would be mounted to a solid test stand and the runups done on the stand well away from the rest of the folks in the pits. And the engine only advanced to full throttle once standing safely behind the stand and reaching foward gingerly...

One should not in any way underestimate the power of prop blades of that size. They could easily cause serious injury or even death at closer range.