That is one of the things that technology has done to us. The ability to hear music in our heads has grown very week with the invention of instant playback. And at times and can feel limiting when, writing for live performers, when you have to edit your music to fit practical instrument limitations. However, this shouldn't limit your musical visions. Through understanding instrument limitations and knowing how to write good orchestrations you will find that you can still write almost anything you want (except for those really really impossible things like 64th notes at quarter note = 270 and what not)
I personally have never feel that the out come a live performance doesnt match what I had in mind, nor have I felt limited by the abilities of live performers (unless I am writing for a particular group of performers like in a commission). I did feel this in the beginning because like most young composers, I too was spoiled by perfect playback. But after years of hearing live performances I have trained my mental ears to hear my music as performed by live performers. For me, it was a crucial skill because majority of my music is intended for live performers. Understanding the difference between what I would like a piece to sound and how I would really sound has helped me make informed decisions about my music so I dont limit myself musically.