So if I'd bother to sell some alts in order to link say 5 BTC I'd get 312,5 MB (0.31 GB). This would be currently worth...0.024 BTC (~$24). Correct?
Let's say yes

.
Quite a modest amount but perhaps it might be worth it.
What also might be worth it, would be to buy GB instead. Of course there's more risk involved as you would invest your BTC instead of just linking it.