How much of an Improvement would be had from swapping the 853's on my 99 SS to some stock 241's? All comments welcome

 

Stock 241's? None. Stock 243's? ~15hp. Depending on your application, you could just spend the money on porting the 853's and pick up closer to 20-25hp.

 

mk well that doesn't seem worth it. Guess ill just save up for some real heads lol

 

You talking just heads or heads and cam? If already cammed I would expect larger gains, and if doing a cam at the same time huge gains.
Properly ported 243s are great for stockish displacement engines and many strokers.

 

Yeah I would be doing 243's with a cam at the same time.

 

I feel like you would have to spend some cash to make an 853 head pick up that much power. Larger valves and the whole deal. I have seen 243s go for less then 500, between the small hp gain and bump in compression I think its worth it if you can find a good deal. Also 243s respond pretty well to porting, you can get them ported later on down the road and pick up even more power.