The  "theories "on down thrust  are  pretty darn subjective
 A  LOT depends on the  speeds yo like to  fly.
   Why the larger engine?
 A whim?
 essentially th model is  of  a size  and the  wing setup makes for  a lot of  climb under power
 The  huge down thrust  was likely done to  increase speed yet   hopefully provide  some force to  keep the  nose  down
You can screw around with the  speed n downthrust  and the wing saddle position till the  cows come home 
 BUT the  stock setup makes for  a gentle very ,very easy model to fly.
 My old Seniorita can quickly be trimmed  for  complete hands off  flying - it  will  do huge circles  with a gentle undulating  flight path
 meaning the model will slightly climb till speed drops back  then dive slightly  picking up speed  and  the  nose will gradually  come up  -repeat etc.. lovely  free flight  flying
 If you are after an aerobatic setup , the  basic model arrangement is  wrong  -so once you dig into  more  power for more speed  you wil end up having to  change  angles of  wing and thrust  -or just accept it is what it is.