I am a little confused by your question, but let me see if I understand what you are saying:
You have a product page, then you have any accompanying pop-up for it, and you just changed all of the URLs for the pop-ups and the old pop-up URLs are redirecting to the product page?
Were you getting a lot of people linking to the pop-up URLs? That is really the only reason you'd need to 301 them (in my opinion). If not, then ONLY redirect the popular pop-up URLs. This would definitely help with load times.
And your dev team is probably correct. If you have 100,000 lines of redirects, that is going to slow things down. Depending on your environment, you could potentially set up rules that would redirect things; however, it may be difficult depending on your URL structure/pattern.
As far as 410 or 404, a 410 usually alerts Google to remove the URL from its index faster than a 404, but in the end they essentially work the same.
Hope this helps,
Mike
