Hi bear - I had this with DS, very similar, he was 8lbs at birth and in day 3 had lost 11%, I was advised to top up each feed which I did. I would bf for up to 45 mins on each side but still have to give 2oz formula afterwards.
When he was 1 week old we had tongue-tie snipped, I noticed an immediate improvement to latch and my own pain durning BF subsided a lot.
I was advised to keep him on the breast pretty much constantly which I did. Despite this, and despite the fact I had never given a top-up without BFing first, my supply never really came in/ recovered. I subsequently did a lot of pumping to try to address this but it still didn't really work and at one point (at about 3-4 weeks) I was either BFing or pumping with no break at all and I found it very tough.
I persevered with BFing for 8 weeks at which point my supply was gone altogether; DS would go on the breast for 30 seconds then come off and scream.
I will never know if my lack of supply caused the initial weight loss or whether the top-ups negatively affected my supply. My logic says possibly the former as even when topping up I had the baby on the breast so much that my supply should have been stimulated anyway. But who knows.
However, if I had my time again I would not top up on day three, I would have given until after the TT snip (although TT was not diagnosed until day 5).
I guess if they've asked you to wait until Wednesday then that's giving you a chance and if weight loss hasn't stalled by then I probably would give a top up.
Hope the above is of some help- it's so hard to know what to do isn't it? I'm expecting no.3 in a week and a half and although I'm determined not to get bogged down over feeding I'm sure I will!
Very best of luck with everything.