Thanks for the feedback.
At this stage, eyepieces like TeleVues are luxury items I can't really afford/justify. Maybe in the future if I get serious enough to splurge on a SCT.
Trawling the web, it appears that the GSO Superview 5-element eyepiece is an Erfle design; there were several complaints about problems focusing with some telescopes (requires more out focus than the 3-element Kellner), and the very long eye relief can cause "blackouts".
I'm pretty satisfied with the 26mm Kellner that came with the scope, so I'll give the 40mm version a go even if I end up replacing it later.