This new mulligan rule without Serum Powder or Scry is better than the current rule with Serum Powder + Scry
New rule: 4.7% chance of miss = 95.3% chance of hit
(should also consider the top card of the deck if you're on the draw, with 6 known misses on the bottom of the deck)
Current rule, with Powder + Scry: 94.2% chance to hit during mulligans, 95.0% chance to hit after scry + draw.
So this new rule would be an upgrade without Powder. With Powder the math gets complicated (you could exile multiple Serum Powder at once) but Powder should eliminate ballpark 2/3 of the mulligans to oblivion. In this case, the miss rate would fall to about 1.5%. My suspicion is that because you always Powder for 7 under the new rule it has a little more impact than that and you get the miss rate down to 1%ish
In short: yes please
One source of the mulligan numbers is here: http://www.archive.themanadrain.com/index.php?topic=48196.0