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I am considering building a 1380 eight port but using longer than standard S rods ( +1/4" ) and then shorter compression height pistons 1.248" as opposed to 1.498" compression height.

What experience do others have of this configuration and the benefits experienced?

The idea is to reduce piston slap at high revs by putting the gudgeon pin higher in the piston and lengthening the rod. Personally I wouldn't worry about it for a road engine as you're unlikely to reach the kind of revs where slap would become an issue.

THAT is not the main reason for doing it!

well one obvious benefit is to reduce the rotating mass in the engine , any others , am also of the theory the rod geometry improves also .....

researching the concept before I decide which approach I take.

I'm sure a few other will jump in with the pluses on this type of set up, however, be mindful that increasing the rod length also increases the Rod to stoke ratio. For a five port head, with Siamese inlet, like on our A series a reduced rod / stoke ratio will have HP benefits.