I always get upset when a DM suggests that a player is getting too powerful with a perfectly logical, legal combination of abilities. No. They are not too powerful. They are simply ready for more than you are giving them. I threw the freaking Demogorgan at my players at one point because I knew they were powerful. Up the anti. Make your monsters fight better. Rejoice that they have figured out something powerful, because now you can bring about something even more so. Let them shine, and then knock them down. If it becomes that big of a deal, have the baddies target the steed. Make a baddie who curses the steed so it becomes sickly and unusable, and force them on a quest to heal it.
I had a DM who allowed my Rogue get a hold of a Shield Guardian. Big mistake. Went through the entire Tomb of Horrors with only me, a partner, and Herman, my Shield Guardian who wore a Cap of Disguise to look like an Orc Bodyguard wearing a tuxedo and sunglasses. Killed the Lich by accident, because a room with a bunch of huge statues and a portal that apparently led to the Vault, we got stuck and in a rage I had Herman throw all the statues through the portal. Which went to the Vault and killed the lich for us. Didn't even realize it until we got there. It was awesome. We felt like kings. Especially since fighting the Lich with only the three of us would have been impossible at level 8. Totally unrealistic, but we had a blast. Took the money and built a castle, had a town grow out of it, and when we played another campaign later, that town became a base for our new characters.
I should NOT have had that Shield Guardian, it made things way too easy. But it was fun as hell. Especially since I, having a Rogue, always seemed to figure out a way out of the problems, usually without combat. And Herman is legendary in our group. My sword was a Frostbrand named "The Common Cold." "You have been killed by The Common Cold" was my tag line.
Let them play. Let them feel awesome. Don't worry if other characters don't feel as awesome, let them find their own way to be awesome. Help them along if you have to, but don't nerf someone else because they play better. I promise you it will be ok.
Edit: Oh I forgot, Herman died in the Tomb of Horrors. Getting to the last room, turning the key makes the floor rise up into the ceiling. You're supposed to get out of the room, and everyone made it but Herman. He got squished. My DM was so relieved. It was a fitting end.