Very often in the middle of combat something may happen that might improve your initiative, such as an increase in Dexterity... when this happens, we adjust their place in the initiative order appropriately, giving them the opportunity to get the drop on someone so to speak.
In other words, if everyone rolls initiative to open combat and character A gets an adjusted roll of 14, that becomes her place in the initiative order, ahead of anyone who rolled 13 or lower and behind anyone who rolled 15 or higher... but in round 2 she is subjected to a Cat's Grace spell - well, we move her up to 16 in the initiative order, even if it means she'll get a chance to act twice ahead of someone who may have been just ahead of her in the order.
Has anyone else tried this or something similar and if so how did it work for them?
No I can't say I've used this but this strike me as potential abuse of a turn based system.
Sounds like it could be a fun modification. I wouldn't allow a player to act twice in a round though. Maybe have the new initiative take effect after everyone has acted in the round.
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Whatever initiative you have, you cannot act twice in the round.
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In your example, A has Init 14. Let's only assume simple initiative ticks, for sake of simplicity.
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If she receives a Cat's Grace on a 16 or higher, all you do is move her initiative up. She has yet to act in this turn, and will do so at 16.
If she receives a Cat's Grace when she has already acted (e.g. at 13), move her up to 16 and have her act at that 16 from next round on.
The only tricky situation is if she has not yet acted, but the buff shifts her Initiative to a higher value than the current tick (e.g. on tick 15). In this case, I recommend to have her act right now, then adjust her initiative up to 16 for any further round. Still, you only act once.
Yes, this can mean you act twice between two opponents actions, although in different rounds, much the same an opponent can act twice between two of your actions if you used to be faster than him, then delay.
(For those who didn't follow...
Round 1: Monster (15) - Char (14) - INITIATIVE BUFF!! (after 14)
Round 2: Char (16) - Monster (15)
resulting in an action sequence of Monster - Char - Char - Monster).
Oh, well if all anyone is doing is increasing their Initiative then I guess it's fine.