Is the Manley Steelhead still relevant?

Yes, the Manley Steelhead is still relevant. I don´t see how you could do better for the money.
A caveat: I believe that the earlier model had QC problems, but these seem to have been overcome.

Ask member Lewm.

I've owned mine for five years and I love it. Each time I have improved some aspect of my turntables I have discovered just how good the Steelhead is. Changing from VPI decks to hot-rodded NOS vintage decks with Reed 3P arms. Ven den Hul Crimson Colibri. Brian Walsh doing his thing. 
The Manley Steelhead will not hold you back. I don't claim nothing beats it for the price, but I do claim it would be next to impossible to think of it as a limiter/bottleneck. 
Btw, many claim to have had hum problems due to the Steelhead and imho they are wrong. Once I jettisoned my two VPI arms, my hum problems went away. De nada. Zilch. Bupkis. 
I don't doubt Mikey for a second insisting that for his system, newer more elaborate designs beat it. But my system is pretty "up there", just not Mikey level. 

I ran a Steelhead RC for four years, during which time I went through four turntables, five tonearms, and many cartridges. I liked it a lot; for the money and versatility, it's hard to beat. But that said, it was readily bested by the Allnic H3000 that replaced it, albeit at a substantially higher cost.

$15K will get the "end game" piece.

As mentioned above, the Allnic, or something cool as the Luxman are at that level. After that, it's all table madness to blame for nervosa!