If you want Xp to be shared with rule 25% per party member (in that case maximum party is 4 players!), there is no difference in it, than in kill order, which is practiced in group hun currently.
Mobs, that are spawning in encounter are random rolled betwean 1 and 5 from each group (I mean those better XP mobs). This mechanics (spawning) combined with your suggestion would end up in exact calculations (time x XP) as it is now.
However, better slution would be to add BONUS of 25% XP gain for every kill made in team for other party members. Example: I kill monster, I get 1000XP, all my mates gets additional 250XP for being in team and helping. If other member kills said monster, he gets 1000, and I get 250XP.
Also, rule of mob spawinng in ecnounters while leading a party should gain new rule with: 2 Base monsters + 2 for every party member (so party of 5 would be granted with 12 monsters - which, itself, would generate more XP from encounter.
Curently all mobs spawn in same manner, so rule its always TIME x XP / Party members - and should be like: Time x XP + Party Members