Even so it has base 90 speed and 120 atk, although not 130/102 like garchomp, it has base 100 df and 150 hp, making it more bulky than garchomp and able to take more hits while outrageing. Not to mention of it gets fireblast it would hit much harder against steels. Even if it doesn't get fire blast, draco metor would still be better than garchomp. Scarf, and it basically beats most pokemon to the punch and wins.
Fire Blast is a Tm and Giratina can't learn it. Alternate forms so far have the same TM move pool. Also what steels are you trying to hit, Metagross, Jirachi? i think garchomp is happy with using earthquake.
All that HP is a waste on an offensive giratina, 120 At is pitiful in ubers and when you are weak to the most common attacks in ubers and have a base speed of 90 it makes it harder for giratina to work. Garchomp is fast enough to outspeed deoxys-F with a scarf and can out speed Scarfpalkia which is extremely common now.
If you are using a mixed set or special Draco meteor set, i already said Dialga pretty much outclasses giratina in the offensive side, and dialga also learns Fire Blast.
Though assuming he does have the above stats, they aren't confirmed yet.
I don't believe there will be a light clay of TR, as it would mess up trading and such. Emerald had plenty more keyitems than r/s (the key item that opens the team magma hideout south of mt.chimney or whatever its called. It is before lavaridge town.)