That will definitely take away some strength, unless you weld extra plates over both parts being welded together. That can actually make the seam stronger, as long as the person welding it knows what they're doing. Don't rely only on a butt weld.

That being said, I replaced everything in front of the strut tower after having a head-on collision with a telephone pole (black ice). It's not too bad as long as you plan out everything in advance. I just went to town with a sawzall and and then had a hard time getting the new and old pieces to line up correctly. Don't eyeball it. Take many measurements BEFORE cutting the good car. Use a compass or inclinometer (assuming you're doing this on a level surface). Make a drawing with exact measurements.

Let me know how it goes if you do this. If it goes well, I might look into doing it to mine.