Hi Chris, the Jantzen cap you have chosen sounds great and is a nice upgrade over the SCR (Solen) in your photo. I can't see the rest of your XO but if it uses those white sand cast abominations for resistors then while you are at it replace them too. Mills 12W are good but not the only option. I see some wires crimped to a brass push-on lug. I recommend cutting the nasty thing off and soldering instead.
Don't know if you are aware of this very informative/exhaustive cap collection review. I find it to be accurate regarding the caps I have tried.
http://www.humblehomemadehifi.com/Cap.html
Do not be swayed by those who think reproduction of the very high frequencies are unimportant because they can't hear above say 14Khz. It's the higher order harmonics that contain much of the clues to the acoustic space and the placement of performers within that space.
The higher frequencies when reproduced combined with the instrument's fundamental allow the ear/brain relationship to identify that instrument. If the harmonics are removed from some instruments like piano, sax or any other and are asked to play for example middle-C, what you will hear is a pure tone of 261.6 Hz. That is to say you will not be able to tell what is producing that annoying tone. When the instruments complex harmonics are added back to the fundamental you will then be able to recognise the instrument as a piano and the more complete the harmonics the more the piano sounds like a piano and the more delineated it becomes. This leads to less listener fatigue.
In nature we tuned into these high frequencies as part of our survival mechanism. A sharp crack of branch or dry twig snapping would alert us to danger and also indicate the direction from which it may come. Now if all the old farts could not hear the top octave as part of the sound they would have all been eaten.
Look up a white paper article by Max Townshend who is a proponent and producer of super-tweeters which respond up to about 60Khz or somewhere. Can't remember the title but something to do with 'Beyond 20Khz' A very important disclosure and a way of taking your sound beyond what cables and amp upgrades can achieve.
The Miflex caps I have not heard so perhaps you would like to buy a pair of those as well so you can report back with your findings