Q:

Would using a lower-leveled spell for the a Warlock's Mystic Arcanum feature impair balance?

I confess I find myself underwhelmed by the 8th level spells available for a Warlock's Mystic Arcanum feature, especially if you e.g. have one or more bards in the party (as I do).
(Maddening Darkness seems like it has such an enormous radius it'd disable your party, Glibness and Dominate Monster seem covered by bards, Power Word Stun and Demiplane look underwhelming, and I'm not sure how I feel about Feeblemind -- I saw a claim that casting it was arguably less ethical than making your pact in the first place.)
Honestly, the 7th level spells all look much more interesting, flexible, and useful.  (Etherealness, Forcecage, Crown of Stars, Plane Shift, and Finger of Death all look like choices with significantly greater flexibility or suitability to fill niches bards can't cover, especially the blaster role.)
I see from this question that it is not RAW-compliant to choose a 7th-level spell (instead of an 8th-level one) for their Mystic Arcanum at warlock level 15, but would this be unbalanced?

A:

With the currently (03/2019) released spells, it's balanced.
Non-warlock full casters can cast their spells with higher-level slots. Since the level at which warlocks gain Mystic Arcanum spells of level X is equal to the level at which non-warlocks get level X spells and slots, they can't cast spells of a given level more often than non-warlock-spellcasters - with or without your house rule.
For example, a level 15 wizard has one level 7 and one level 8 slot. Therefore, he can cast 2 level 7 spells a day. If a warlock can now cast 2 (different) level 7 spells per day as well (instead of a 7th and an 8th level spell), it shouldn't introduce any balancing issues that don't exist with other classes as per RAW already.
Warlock-exclusive spells might be an exception here, so we'll have to take a more detailed look at those.
However, there are no Warlock-exclusive spells of level 6 or higher, so it won't be an issue. Even the rare case where two specific spells would not otherwise be available to the same caster is negligible since bards can get any two spells from any spell list at level 18 (Magical Secrets class feature).
Since all warlock spells are also available to other casters, who do have the ability to upcast spells, it's also not unbalanced to allow your Warlock to choose upcasted spells for his Mystic Arcanum. For example, it would be balanced if for your level 7 Mystic Arcanum, you could choose the 6th-level spell Circle of Death, but it's automatically upcast to level 7.
Besides, when looking over the warlock spell list, I don't see any obvious balancing issues that arise from knowing multiple 6th-level and higher warlock spells of the same level.
If anything, it gives your players more room for creativity, for example by escaping from a Forcecage they created themselves (although there's rarely in point in doing that), or by casting Feeblemind and subsequently Dominate Monster (for a pretty much guaranteed fail unless your DC is super low) on the same creature - which could also be accomplished by a single Bard or Wizard.

