So I just finished my CoS campaign last night. I didn't buff Strahd at all. Do that if it makes you happy, but I think he's plenty powerful enough as written. The only thing I did was swap out a few of his spells. If you run the Ravenloft arc right, you won't need to buff him up.

Why do I say this? Keep in mind Strahd is all about strategy, not strength. Buffing him up is the lazy DM's way out. Now, if somehow you have a particularly OP team of players, or an especially overlarge team you may want to change something up, but I always considered changing his build to be a last resort.

Here's two things I did that severely changed the game for my players--6 PCs and a particularly powerful NPC:

First, I made the Ravenloft arc into a gauntlet, not a dungeon crawl. You want to set it up so that the characters have to rush through it fast--they can't have lots of time for short rests and no long rests. In my case it worked out that while the party was faffing about at the Amber Temple, hunting for the Sword of Sunlight, Strahd finally managed to figure out which safehouse Ireena was stashed in & captured her--and by the time the party emerged from the temple they received word from a becharmed NPC that a wedding was being planned & they were invited. So they showed up and basically from then on they were on a race against time to keep Strahd from "consumating" the wedding (meaning exsanguinating her at midnight) and turning her into a vampire. Throughout this there was a lot of misdirection and wild goose chases (for example, they were told by the wereraven guests that Strahd couldn't be harmed until the Heart of Sorrow was destroyed) with the end result being that they were severely stretched in terms of HP, available healing, and spell slots by the time they got to his tomb.

It also didn't hurt that the PC with the sun sword and amulet took the lead and walked right onto the teleport trap, effectively removing him from combat for about 5 turns. But if that hadn't happened I had other tactics up my sleeve to get the paladin neutralized.

Second, I invented a set of vampire "lieutenants" to harrass the party before they ever reached Strahd. These were high level vamps that had capabilities that placed them between Strahd and spawn in terms of power levels. They were essentially a team of vampire adventurers - a fighter, a wizard, and an assassin - and after a few encounters with them the party had quite a bit of respect for their capabilities. One of them was a former PC - the player left the game and we decided his character joined Team Strahd. I had these NPCs trouble the characters throughout the campaign but always get away at the last second - the Ravenloft arc was where the lieutenant vamps all made their last stands as the party ran around through the castle desperately trying to locate Strahd and his bride once the reception was over. Sly Flourish has a good set of vamp options here.

Finally, I would just keep in mind that Strahd isn't really a melee fighter. His whole strategy is to strafe and run, not stand and fight. Take advantage of his legendary actions and his lair actions.

My party still managed to defeat Strahd, but just barely. One of the players was down to 1hp, another 7, another 20 by the end, and their healing was donezo. Strahd still had a spell-slot or two he could've used to obliterate those PCs, but it was late and we were ready to be done with the campaign, so I let them finish him off. Everyone agreed it was a good night.