Per the OP (!) the MRW-HF100s are base loaded whips not dissimilar to the majority of garden variety CB radio whip (and I have to slot Elecraft's spendy examples in the same category.) In resonating the random length telescopic whip, much of the useful current is swallowed up by the base loading coil which will either be lossy, not so lossy or extremely lossy. The query really should be about the extent of the loss in those coils which I'm not able to answer, but looking at their lengths and diameter, I'd hazzard a guess that they are not of the most efficient type.
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I was tempted by this antenna too, but head had to rule heart and take the view that as being ?100 better put towards something else. Perhaps if they'd slash the price I'd buy it out of curiosity in the full knowledge of what I was entering into, but (eg) an Icom 7300 at- what- I don't know, ?1300 (probably less?) against that antenna for ?100? It just doesn't add up. This accusation could easily be levelled at any number of ham radio peripheral doo-hickeys.
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The best approach is the "glass half full" one- which is to eschew WSPR and to judge it against the number of contacts made which upsets the folks reading eHam reviews so much, but it is at least practical. What's the point of knowing the "Q" value of those coils to several decimal places if you can get a few contacts? (if!)
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Knock up a re-configurable "L" match and throw out some wire!
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Steve, G4VRR.
Steve