So, here's the gist of it

Erwin planned to syphon all the energy from the Spectral Realm to grant his machines more power.

Doing this would cause the spectral realm and Pac Man's realm to collapse

Erwin is intending to use the energy to power his robots. The final boss fight was powered by this, and Pac Man took hits from it (additionally, there is also no reason for his combat robot to be weaker than his Syphons anyways, which was able to pull in and maintain that energy, as well as extract it at that speed. Worth noting that Erwin takes great pride in that combat mech). Erwin even states that it could survive the realms collapsing

Pac Man survives the explosion of the spectral energy. Although he is teleported back to Pac Land, he makes a sound when he is teleported, which is not heard here.

The syphons extracting the spectral energy into the real world is a speed feat. This is because there is a syphon in the spectral realm which pulls in the energy from across the entire dimension into a portal which is then contained within the real world syphon

The timeframe isn't very long. Before he went to Gogekka, Pac Man was in the spectral realm when it was fine. Once he finishes that level, he goes right back to the spectral realm, where everything is on the verge of collapsing due to the energy extraction. I'd recommend using radius of the universe, and 20 min - an hour for a timeframe.

Universe radius: 4.4e+26 Meters

1 hour

4.4e+26/3600 = 1.2222222e+23 m/s, 407 trillion c

20 minutes

4.4e26/1200 = 3.6666667e+23 m/s, 1.22 quadrillion c

The Spectral realm should be universe sized.

For starters, it visibly has cosmological objects and stars in the background

Second, the spectral realm is not an artificial universe. It is an organic one, and never treated as inferior to the real universe in size. I don't understand why the size should be in question if it is organic and has these large objects, nothing implies a smaller size. Hell, if we question the size of the spectral realm regardless just because it isn't the "main" universe, then why not question the size of the main universe itself in every series?

Just to note, this would not actually affect the uni-low multi argument, as it was going to destroy the real realm as well.