You have a SI and I don’t know exactly how your electric windows are wired but at least on the SIII the power to the windows goes through the switches resulting in a very long back and forth of cables and a switch that often gets overloaded.
The end result is that the windows motors never see 12v, rather 9-10v if you are lucky.
With the relays you use the existing cables for the primary circuit, i. e. to drive the relays and you bring a nice and juicy new cable from the battery to drive the windows.
And if you put the relays as close to the windows as possible you will have minimal to zero loses.
I went somewhere in between and put them in the center console, it was the best compromise between function and having to take the whole interior apart…
The difference in the windows speed was very significant.
Aristides