There are three different things going on here.
First, the Comp Thumpr cams came out at least 6 months before Hughes' Whiplash. I believe that the Whiplash was a response to the Thumpr and in typical Hughes fashion they did it just a little better. As you yourself noted the Hughes "real Chrysler" cam has more lift per degree of duration. It sounds like the Comp is one more Chevy lobe on a whatever core.
Also, the quicker rate of lift for larger diameter lifters is not a Hughes secret, it's an early Machine Design principle. So to say that the Comp is in any way a ripoff of Hughes is laughable. It just took Comp a little longer to see the market for the lobe, they probably let Hughes do their market research for them. Again, this is common in the competitive world of industry.
Okay, now on to second point, you may have old springs where the part number stayed the same but the spring itself was changed slightly (upgraded) over the years. So I'd believe what's on the box they came in. And yes, springs are blessedly linear and your math looks right.
Third, I am about as cheap as they come and the more I think about this the more I think you will be all right without spending the extra $$$$. Keep in mind to break the cam in to Hughes specs, including removing the inner springs. Then be careful in the upper rpm ranges where the springs might possibly allow valve float or bounce earlier than a stiffer spring.
As for the lifters, they all come from just a couple of places, as long as the oil doesn't come squirting out around the lifter it should be OK.

Keep us posted on your results...
R.