How are the growth charts? Exclusively breastfed babies have their own growth chart but I never did bother to chart my daughter's on it.
I exclusively breastfed (save maybe 4x when I tried the bottle because of your exact same situation - feeling the baby wasn't getting enough) until just last December. My daughter will be 3 in April. She used to nurse for 30-60 minutes as an infant so I guess my flow was slow or she wasn't sucking hard enough yet. And she has always been in the single digit percentile for weight but always 75%+ for height. "Tall and thin" is just how she is was what the pediatrician always said. I didn't have an oversupply issue and I was definitely eating a lot to compensate for breastfeeding calories.
So long as they are peeing and pooping what they are supposed to at their age, I would not be concerned (in hindsight). I was like you too when I was going through it! You could try squeezing the boob while they are latched to get faster flow. Also you could express a little first before they feed so they get the hind milk (the fattier milk sooner). That is what my lactation consultant suggested to me, not a bottle. Good luck!