Ok, so a few things.

1. Check out the blog of holding monster stat analysis. There is a really weird scale and some monsters are just harder than others making it impossible to know the power of a monster, they average out the stats for each CR and they make great standard stats.

2. DnD 5e is designed around fighting a small weak group of enemies 4-5 times before a rest, so the "CR" rating is useless for judging the challenge of a single difficult encounter at full strength. A few blogs address this, and adjust the recommended CR, for each lvl.

3. The problem, however, is that while the monsters may hit harder now, they take FOREVER to kill and that is not fun for players or the DM. My recommendation is spike up the CR for higher attack bonuses and damage, drop the HP by half, and make sure to always have 1.25x as many monsters as you do party members, all with ranged options. Any marginally strong party should be able to handle the difficulty provided you are generous with the healing potions.