I'm going to build a fly rod on an 8wt. TiCrX 4-piece blank. I'm planning to use an REC reelseat cannibalized from a broken rod. This seat has an internal, screw-out fighting butt which will add 3" to the length of the completed bottom section of the rod. If I trim 3" off the butt section of the blank to make all four sections equal in length will it significantly degrade the performance of the rod? I worry that foolish consistencey will make me look small.

Nope, won't hurt a thing. It will however, make the rod 3" shorter if you use it without the fighting butt (which probably isn't going to happen seeing as it's an 8 wt).

You'll lose a little power - it'll be like fishing a little shorter rod than a standard 9 foot blank (I'm assuming this is a 9 footer) because were you locate your fighting hand matters and it will be 3 inches further forward than usual. But overall, I'd go with what Ted said, it really won't make all that much difference.

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Yes, a little power loss, but in the end with the most, and by the time you're into that part of the rod the difference isn't all that much as it's the stiffest part. So, as Tom said, it'll basically be like fishing with an 8'9" rod, butt power wise. If you were to have a fighting butt the same length built on the blank, no difference. Have to say, a 3" fighting butt is a bit long. Trim away, permanently install fighting butt, and you're good to go, plus all sections will be the same length for the travel tube.

I've built an 8, 10 & 12 wt that I built a 2.5" butt directly on the last 2.5" of blank. They work fine, I saved a tad bit of weight not using the male and female adapter. On those size rods I seriously doubt I'll miss the removable butt option.

If your rod case will accommodate the extra three inches, why not allow the butt section piece be three inches longer than the other pieces? That way you won't lose what may be a valuable three inches of mending length.

Jeff, I think I am going to follow your advice. The REC reel seat in question has an internal, non-removable extension butt which firest unscrews, then extends, then threading in the same direction locks into place. Your comment makes me think that the extra leverage the extention butt gives me fighting a fish may also apply to the extra leverage those 3" give me casting - especially while wading.
Thank you all for your input.

If the seat id the RSLL or RSLS you can simply remove the female threaded insert by heating the epoxy holding it in place and build as usual.
I built two "identical" blanks - one with no extended butt and the other with the extended butt. The non-extended butt had a crisper action. Depends what pleases you. But again, if you wish to build a 9' rod - just remove the sucker.
Herb