even if the Mier/Oehlbach cable is close to the other more exspensive upgrades, (which from all accounts it seems to be) it is one of the best deals a person can ask for as far as tweaking your system and getting that last 5% to really shine.
my impression of the W1000's compared to the HD650 is that what you hear with the W1000 is a slightly exaggerated sound that often times too bright, especially with acoustical instruments. the highs are simply not realistic and therefore make the Senns sound "less exciting." but, imo, this means that the Senns are actually closer to reality, but still with that warm, rich characteristic Sennheiser sound. the 650's also are almost completely free from that nasty thing called reverb where the driver housing along with other elements affect the separation and decay of the musical notes, especially the extremely fast, hard hitting notes wherein the W1000's simply mush them all together. again, imo, the W1000's and CD3000 (i see them roughly in the same family...not exactly alike, but similar in a lot of ways) have that nasal-like over-exaggerated-bright sound that i personally really disslike. i say you're making the right move in giving the Senn650's a good month to see wether you end up liking the that type of sound. if you don't end up liking them, at least you'll be able to say you tried in addition to having good judgement when comparing and contrasting other phones to it.
for an example...
at first, i hated my "stale" ety's with a passion. they were good but also lacked a lot. after about a month of an open minded appraoch (not forcing myself to like them), they grew on me like no other headphone has ever done. now, i hold them as high as and equal to my beloved HD650's!
anyways, i just thought i'd help you draw from another person's experience in going from one sound characteristic to its opposite. it's a big move to make, but if you end up liking it more in the end, it is surely worth it.