If the MLP is much off-center the need for a center speaker becomes paramount for movies & TV. How much off center is acceptable depends upon the soundstage presented by the L-R speakers.
I think this bears repeating.
I have been very happy using a phantom center (as in no center, but "phantom" sounds cool!) where the tv was close to centered between speakers.
Since TV/Movies are not that big of a deal for you, put your money into your primary stereo speakers, not a center!
Honestly, if I look at my own use, it is about 95%/5% music to HT. About 10% of the 5% that is HT, or 0.5% of my total use is content where I would care to have a good HT beyond good audio.
I know some here will consider me a Philistine, but IOW, for almost any TV content, I would be happy to have quality audio from a single speaker (news, sitcom, etc.), having good stereo speakers is just icing on the cake. It is only certain movies and the rare TV show that I appreciate having a HT setup. A movie like "When Harry met Sally" don't care about fancy audio. Something like "Avatar" is where I do want it!
But then, when going from simple Stereo to HT, this is my order of preference for the features of a HT:
1) Deep Base - subwoofers (75%)
2) Surround sound - satellite (20%)
3) Center channel - center (5%, if that)
What I am saying is that, given your use, a center should be way down on your list! Being an engineer geek and using my own preferences which seem not too far from yours, I would put it at 5% of 5% of 10% or
0.1%.
If my assumptions are reasonable, I have a hard time seeing you spend over 20% of you speaker budget on a center!
You've already got nice subs, so I would get a couple of NHT SuperZeros for surround (or whatever floats your boat - as long as they have good mids and highs - the NHT's are small and light for easy wall mounting and will put out plenty of sound for your setup) and go for your final music/stereo speakers before investing in a center. Typically, a center costs more than a single Left or Right speaker (using bookshelf speakers).
By your own reckoning, you plan to replace your L&R with a Philharmonic product that is on par or better than a Sierra 2 w/Raal (I mention this because most readers would consider the Sierra Raal an "end-game speaker", and far fewer people are familiar with Dennis's offerings).
PS - FYI, Dennis has recommended the NHT's as a center (or surrounds) having a similar timbre to his own upscale offerings. Obviously they won't be at the same level, but if someone is looking to keep costs down for the HT part and/or needs a smaller profile to fit under the TV, an NHT is a good solution.