I think they weren't just shocked at Goku's performance against Chappa, but at the expressions Goku had during the
fight, making those feats look like nothing. Yamcha/Krillin could probably replicate those feats, but only if they fought
to their limits.
After seeing Goku's relaxed face at producing a power close to his own max, Krillin tests the waters by saying to Goku
''careful, don't put all your strength in the first match''. When Goku confirms the bald head's suspicions that what they'd
seen wasn't all his power, Krillin/Yamcha were like ''how much power is he hiding, is it possible that we're screwed here?''.
But, till he fought Goku, Krillin wasn't sure how much power Goku was hiding, so he still felt a slim hope.
My gaps are...
Chappa: 6
Goku (Chappa): 7
Krillin/Yamcha: 7.5
Goku (what Krillin hoped to be his max, after seeing the Chappa fight): 8-8.5