Pre-chiller efficiency w/ plate chiller

So I have a plate chiller which works pretty nicely. Brings the temps down to within a few degrees of groundwater temps. Problem is that during the summer the ground water temps are 72F+. I don't want to pitch at 76F. I already have an old 25' 3/8'' IC, which I'm thinking of making into a pre-chiller. Shouldn't be hard, just need to buy some fittings and bit of tubing. In an ice bath at say 34F, how much drop in water temp do you think i'll get by the time I exit the IC? Obviously depends on flow rate, but take a guess. If I can drop it 10-15F or so that would be fine and any more will just mean less water used.

You should be able to easily drop the temp from 75 F by 10-15 degrees and probably more. You will need to stir the ice batch for optimum performance. That will be key to getting the most out of it, even more so than the flow rate IMO. I'm guessing as usual, so YMMV etc.

I use a cheap Harbor Freight submersible pump to feed my IC. It sits in a cooler full of ice water. That way, I avoid messing with a pre-chiller. I do have to top up the cooler.

Before I abandoned a pre-chiller for a post-chiller, my 20ft pre-chiller with 10lbs of ice dropped ~84F tap water down to ~74F (before most of the ice melted) at 3 gal/minute.

The pre-chiller netted me ~78F wort at 1 gal/min. Not terribly efficient but it did work.

All things being equal, my new post-chiller produces ~70F wort at 1 gal/min.

Sorry to wake this zombie, but I wonder who else has done thus and what their results have been. I currently have a 25' 3/8" copper IC in ice feeding a 50' 3/8" IC in the wort. Worked amazing on 5 gallon batches.... On 10 gallon batches- not so much. Using 4 10lbs bags of ice I could only get my marzen down to 60° and rushed it into the fermentation cha!ber to get it down to 48°
I've stayed away from plate chillers because I always assumed you needed a pump and I never wanna bother with them EVER. Seems many gravity feed plate chillers and that I could get into.
Using my 50' IC in an ice bath to pre chill the plate chiller seems like it would work well for me. Just got a big hopsider recently too so not worried about clogging up the plate chiller.
As far as sanitizing couldn't you just put the whole plate chiller in boiling water for 20? What would survive that environment???