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If you added octane booster, that is why it sputters under load & runs rich now- NEVER use it. If you did... change the plugs- They are spent.
However, I don't this is the problem... The problem is that you need to seat the rings throughout your entire RPM range under load. Not sure why you kept it at 1/2 - 3/4 throtlle... It needs to be fully loaded... This isn't an old Dodge where everything physically 'wears' into each other... The machining is very precise so that is not necessary- There is no reason to baby it.
If I were you, I'd take it back to whoever built it; have it compression checked & looked over. If they are as good as you say they are- they will stand behind their work.
For something like this, you are bound to get better advice from others who may chime in. In the meantime; Google how to break in an SR.
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