Having gone this route myself and seeing that it was painfully slow, I bought another 25-footer and tossed it into a bucket of tap water as a pre-chiller to supplement the one in the kettle. Minimal returns.
So, I made a new 50-footer, and I combined the two original 25-footers into one 50' pre-chiller and ran that water through my newly made 50' 3/8" copper IC in the kettle. I start with tap water (about 80 F in the summer here) add ice at about the 120 F point, and get 6 gallons down to 65 F in around 12 minutes consistently.
More surface area on the IC in the wort = faster cooling; all else being equal. 50' is better than 25', 1/2" is better than 3/8", etc.