http://www.mnpctech.com/Fanfilter.html

Also, does this filter contain internal cases noises in any perceptible manner?

(intended use would a Compucase LX-6A19 with the front grill cut out and replaced with this aluminum mesh fan filter. No front fan would be installed)

My guess would be - not very restrictive, but worse than the standard fan grill; no significant blocking of case noise (the bezel grille will have more effect).
Nothing beats first hand experience, but since no-one else has replied, I thought I'd offer my opinion.
I've read a hardware guide that suggested that aluminium mesh filters typically cost 1º to 3ºC warmer case temp, but this would depend on a number of factors.

I would think that the extremely small openings of an aluminum mesh fan filter would contain internal noises better than a stamped metal fan opening (because of larger openings)?

I "externalised" 2 CD/DVD devices from their 5.25" bays to make room for a Silentmaxx hard disc enclosure(very quiet now) Dell quoted £30($45) for two plastic bezels. So I bought one of these 120mm filters which I bent to shape, to fill up the gap plus two , tailored to size, bits of plastic stuck on with double sided sticky pads, either side instead.I used sticky pads to fix the filter. They did me a favour because now I have some nice cool air(no fan) going through the filter to cool the drive. The Dell had almost no front ventillation before.

Before I installed the filter the enclosed hard disc temperature was 29/30 celcius.Now it is 33 on average so it has cost about 3 degrees and about same with case temp.The air is also entering at a slower pace but at least dust wo'nt be a problem.
The noise level seems much the same though.(quiet)

I want to install this 120 mm aluminun mesh fan filter on a cut out fan opening of a Compucase LX-6A19 or Antec SLK3700AMB, both of which already have a front fan filter.

Are others using the included plastic fan mesh filter on their LX-6A19 or SLK3700AMB?

Might wanna' check with The Meshmasta', BlueFront.