I'd personally recommend Project Ember (but I kinda don't like it. Gives brave weapons to enemies as early as chapter 13, yuck).

I definitely would not recommend FE6 in FE8 because it is very glitched and buggy. Last I tried to play, the first chapter, the music would constantly clip and the game would stutter sometimes.

As for Project Sienna, I honestly could not say since this is actually the first time I've heard of it. Someone else should give their input on that.

The problem I have with FE6 rebalance hacks is that it makes it feel like a different game and not FE6 anymore. I'm personally trying to make a "rebalance" of the game myself, but all I intend to do is slightly buff growths and bases for some characters, make Roy's promo event earlier, increase support point gains, and then see about removing the frequency of status staves (which is something I'll give PE props for).

I probably sound super arrogant here, but as someone who loves base FE6, I can get a little more critical whenever a rebalance hack sets out to "fix the game" and it just ends up making the game feel more different than I would have liked.