I've been reading a bit about shuffling a pack of cards via riffle shuffles (those being the simplest to automate) and came to wondering some things...

If you riffle shuffle a pack of cards seven times, you get "almost perfect randomness" - what effects does splitting the deck twice before shuffling have?
ie. i split into 3 piles A, B and C. Then riffle A into B (AB), then riffle AB into C - how many times do i need to do that?

What about if you do it with four piles? A, B, C, D -> AB, CD -> ABCD ?

Is it harmful to do that?

What if the deck of cards has more than 52 in it - does that affect the number of piles it's beneficial to do it with?