I've got tiny fruits on all my overwintered chilli plants now - so it's easy to see that the ones in the polytunnel border don't like it - they are definitely behind.
I think this is a root vigour thing so if you can get your soil much lighter it might not matter.
It also depends on how 'happy' the species is to overwinter - starting with the 'just barely':
My Habanero Arbol plants are the best C.annuum I have found (next best was 'Black Pearl' last year), but they produced nothing in the first year and needed to be inside the house from the end of Sep, stripped almost bare by Dec to survive - and they definitely sulked in the spring - even in conditions where seedlings of the same variety are romping away. In the polytunnel the ones in rich soil have hardly moved (no flowers), the ones in equally richly fed pots alongside have 7cm green fruits. No point trying this variety outside.
It's as if the roots have no oomph and can only cope with pot compost lightened with perlite...
My sweet aji 'Dedo de Mocha' plants (C.baccatum) were very late to produce fruit, and preferred a sunny windowsill over winter, but some survived nearly bare in citrus conditions (4C) - though at least half didn't.
They all have flowers now - even the pots outside - but in the polytunnel the ones in soil are a week ahead and the ones in pots alongside are two weeks further ahead.
Incidentally, my next favourite baccatum is 'Bulgarian Teardrop' but it is so early and productive from seed there's really no point in overwintering it.
The real toughie is C.pubescens - manzano/rocoto. These produced nothing in their first year but were perfectly happy at 4C all winter with quite a lot of leaves on. 2 out of 8 did die, but they are much more 'lively' in the spring - they all have fruits, both outside and inside, and it's difficult to say if the soil bothers them or not.
Notice that all my successful varieties are late ones: a) there's not much point overwintering them if they aren't. b) they won't have expended all their energy on cropping before they face the rigours of winter.
If you really want to overwinter an early variety then it might be worth sowing late - probably not this late though.
Hope this helps.
Cheers.
