Please try
   ModPagespeedImageRecompressionQuality 85

See 
https://developers.google.com/speed/docs/mod_pagespeed/filter-image-optimize 
for more details.

I've tried with compression set to 85 - no change.
I've tested also with ModPagespeedImageRecompressionQuality 50 and still, no 
change.

I've added google plus logo as a second image, since first image is pretty big 
and easily surpasses 2k inline filters limit - i thought this could block it.
But for g+ its the same - no changem and smushit.com saves 16bytes (1.73% 
savings).

Ah, I see, you have an image that's *already* been inlined as a data: url, and 
you would like mod_pagespeed to re-optimize that image.  We used to do this, 
but stopped doing so as we ran into bugs when the optimized image was larger 
than our inlining limit!

We should be able to do this, hopefully without the bugs this time around, but 
it's not a big priority for us at the moment.  If other folks would like to see 
this working, give a shout, as we've mostly been ignoring this due to greater 
demand for us to do other things.