Finally, a Reshiram team. I respect that :)
I can make a suggestion for you that can help out your matchup vs Kyogre and even Xerneas.
Remove Heatran and add:
Ferrothorn @ Leftovers
Ability: Iron Barbs
Level: 50
EVs: 100 HP / 228 Atk / 180 SpD
Sassy Nature
IVs: 0 Spe
- Gyro Ball
- Power Whip
- Leech Seed
- Protect
I know it's weird to use this on a Tailwind team, but having Ferrothorn as a backup mon in case Xerneas gets the Geomancy up is pretty nice. The bulk allows you to take a LO Mega Ray's Dragon Ascent and retaliate back with Gyro Ball, while the attack investment OHKOs 0/4 Primal Kyogre with Power Whip. The rest goes in to special bulk.
Your team could also use a secondary Tailwind user in case yours runs out and Whimsi is dead. I recommend not running Specs Reshiram and running LO Tailwind because being locked in on a move that certain Pokemon have an immunity towards in a potential 1v1 situation (Kyogre vs Fire STAB and Xerneas/other fairies vs Dragon STAB) means you automatically lose. With that, you'll have to give Thundy a new item. You can opt for a bulkier set with Sitrus Berry.