It' s sounds like your Thunderheads have been good to you........stick with them.
I am going to say this as politely as I can........A little bit of research on you or your sons part could have prevented 2 lost injured or dead deer. Live deer are not for testing out heads to see if they work.
Couldn' t have said that better myself !!
Your not pushing enough energy ! Spitfires are not a good choice for low KE set up' s , but for me at 70 # of KE they blow through deer like a hot knife through soft butter ! You need to know your set up' s capabilities !!