No clue about the sound leakage.

Speakers won't play loud enough. How much power does that amp have? Let's say you DO hook up eight speakers throughout the house. When you do, you have to set the impedance matching volume controls to x8, because x6 is not available. Every time you do a x2, you lower the possible sound level by 3 dB. x8 = 2x2x2, so it lowers the sound level by 9 dB. If your amp has 50 watts, then each room will get 50 watts less 9 dB, which is 6.25 watts per room. This is why they might not play loud enough.

I had a problem with amp shutdown because of the second zone on a Denon 3803, and I talked to Denon about it. In the conversation, it came out that they really did not intend for the second zone amp to run more than one pair of speakers. It is not designed for that. In particular, the amps cannot run with a low impedance at very low frequencies.

Impedance matching volume controls have almost zero impedance at zero Hertz, in other words at DC. Measure one with a meter. Around 100 Hz, they are up to snuff. In between, the impedance rises from zero to around 8 ohms (4 ohms for some of them).

If the amp you are using cannot handle loads below 4 ohms at very low frequencies, this type of control can cause the receiver to shut down. If there is no signal at those frequencies, it might not matter. (Some amps check for impedance when they start up, and will not connect to an impedance that is too low. Some just shut down when too much current flows.)

The Denons seem like they could be used for multiroom setups with impedance matching volume controls, but that flaw can come into play. I have used them for several such situations with no problem, but blown one receiver in another system.

If you MUST do this, use a variation of a Zobel to maintain impedance at low frequencies. That kept my problem amp from blowing out, and the tech at Denon thought it would be a good idea, although it had not occurred to her before.