install tiger over leopard?

How long have you been running it on Leopard? I ask because the indexing problem can really slow things down until it's done. Also, how much free space is left on your boot drive? If it's nearing capacity it can slow you down too.

Is your backup bootable?
If so, you can run Tiger on the cloned drive.
I suspect that you have some bad apps on your computer that were migrated when you did the A&I.
Since you have the backup, why not erase the HD and insall Leopard from scratch and see if it's still slow or whatever. Don't migrate anything for the time being.
If it's still slow, it's probably your hardware - otherwise you might start migrating apps back, from the backup, one at a time.
If you really give up, you can simply erase the HD again, boot into the clone and using SuperDuper, simply make another clone on your erased HD in the computer.

Take a look at the Activity Monitor and see if there are any errant processes or some obvious things taking up CPU ticks.
At some point, an upgrade on an older system may not improve performance. My dual G5 seems about as fast w/Leopard as Tiger, but it does boot and shutdown faster. Internet performance, wired and Airport is the same as w/Tiger. I only have 1G RAM, but the system is only using half of that.
I run XP via VPC and see no difference there either.
So it may be that I have also hit the limit regarding performance on my G5, but the additional features, TM, etc., make it worthwhile. Besides, I have Tiger on the other internal HD to boot from.
I would hold off uninstalling Leopard for a while and doing some testing and, as you note, wait for at least the first update.

Look your computer is running slow because like most people your computer did not have proper maintenance prior to to installing Leopard. A computer is like your car it needs maintenance. Now that you have already installed Leopard, it is best to what until the three maintenance programs update for Leopard They are Cocktail, TechTools Pro and DiskWarrior. You may try running the Apple Disk Utility from your Leopard start up disk. In the utility highlight your hard drive and click on the Repair Disk button. This many help you for the time being. You should always have a second backup hard drive with a Full Mac OS system installed on it. You will install the maintance software on both your main hard drive and your backup drive, When you see a problem with your computer you switch to the backup hard and run the maintenance programs on your main hard drive. I have been working on the Mac for 22 year. Good luck!

Did you ever get an answer to going back to Tiger?

Apple Care told me "there is no way to go back to Tiger without an erase and re-install +to guarantee+ +no future problems+ because the Leo OS is interwonen right into the Core of the the Mac where you can not easily get to it and uninstall everything".
Since you have backed up everything just put in your Tiger Disk 1 to start the it. Choose the option to erase and reinstall and it will then begin. Follow the direction and when it brings up the bit about Leopard being on there pick the selection to just wipe it all clean. It only takes about 20 minutes on the Pro Mac.