Ok, I'll put bf, not sure the ghosts are really slowing down in the tunnels, but the 3 (actually 4 !) parts added in the 2nd level of mspac6m look better this way, I wonder if they have a usefulness ? I couldn't find any...
I forgot where I got the rom from, it's been quite a few years already, there doesn't seem to be any update available anyway. The odd part is that in hbmame they call this rom mspac6m2 apparently, well the 2 is a correction of the 1 there is not much use to keep a buggy set then, but it's not me who decided to rename the rom as mspac6m, I found it like that.
Here is a screenshot of what I talk about, the wire-frame looking parts.
I think I got the rom from a site about rom hacks, but not sure.