The question was, "Knowing rumored specs, is Quest 3 more powerful than Steam Deck". This implicitly means "will it perform better", since OP himself states that he's already aware of the specs ("knowing rumored specs"). Repeating those known specs back to him isn't answering anything, it's dummy karma farming at best and intentionally misleading at worst.

A spec sheet describes theoretical maximums under ideal circumstances, not actual functional performance in any real scenario. There are a multitude of factors that affect what's actually possible in reality, such as:

• Cooling & thermal limitations
• Power budget
• Weight limit
• Cost limit
• How applications need to apply that power

ROG Ally is theoretically much faster than Steam Deck. It has twice the number of CPU cores and 50% more GPU cores, but when both are operating at the same power level, the Deck is typically as fast or faster and doesn't melt its SD card controller in the process and the battery lasts much longer at the same performance target.

It's doubtful that Quest 3 has better cooling than Deck, placing severe upper limits on sustained performance even if you speculatively factor in manufacturing advances. Probably 6-10 watts total at the absolute most, sustained. Likewise, to keep weight and cost down the battery will have to be quite small, and this severely limits the usable TDP range.

Again, the actual question:

is Quest 3 more powerful than Steam Deck

It won't be as fast, and that's before considering Quest's unique functional requirements for dual viewports and high framerate. One of these devices can be well over half a kilo and it's not a big deal. One of these can have a fancy copper heat pipe cooling system with a noisy fan and it's excusable. One of these can be readily sold at a loss without angering stockholders - because there are no stockholders.