If you have it set at 104 and 5 people get in then add the jets, the water will most likely cool off a bit to say 101 102, one thing you have to consider as mentioned the outside temp. The colder the air the hotter the water will feel to most, I doubt all of the 5 people will notice a decrease in temp, some may find it to hot and want to sit on the side for a bit.
All that being said yes your spa will probably decrease in temp but running the with the 60 amp should maintain a temp of 101 to maybe 104 which will be hot enough for most.
This would be interesting to try, open the spa and get the 5 people in one by one with out the jets on, just sit for a couple of min and see if the temp drops. Then hit the pumps one at a time pmp 1 on low then high run for a couple of min see if temp drops. Then kick on pmp 2 and watch the temp. Would love to here what it does.