Let's look at 10th dince you brought up 5war earlier
10 Sorcerer 10 4 1st level, 3 2nd level, 3 3rd level, 3 4th level, 2 5th level
Sor 5/War 5 4 1st level, 3 2nd level, 8 3rd level
Both can cast 8 fireballs, sure...
But the dorc casts 5 of them doing more dice of damage and has more sorc points base (10 vs 5) to use.
So, the claim that the warlock 5 made his fireballs better or helped them to be cast is not supported. You got worse by losing the 9 and 10 dice casts.
Now compare it to bards mc 5/5 sorc, the slots math remains the same for fireball but the bard suffers the same 5 sorc pts drop. But now we are comparing the bard class and sub-class gains vs the warlocks as well - did not have to sacrifice the high end slots
Right, sorry. Got carried away with even split. If we wanted to see just what impact 5 levels of Warlock has:
[TABLE="class: grid, width: 100%"]
[TR]
[TD]
Level[/TD]
[TD][/TD]
[TD]
Spells[/TD]
[TD][/TD]
[TD]
Spells[/TD]
[/TR]
[TR]
[TD]6[/TD]
[TD]Sorcerer 6[/TD]
[TD]4 1st, 3 2nd, 3 3rd[/TD]
[TD]Sorcerer 1/Warlock 5[/TD]
[TD]2 1st, 6 3rd[/TD]
[/TR]
[TR]
[TD]7[/TD]
[TD]Sorcerer 7[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 1 4th[/TD]
[TD]Sorcerer 2/Warlock 5[/TD]
[TD]3 1st, 6 3rd[/TD]
[/TR]
[TR]
[TD]8[/TD]
[TD]Sorcerer 8[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 2 4th[/TD]
[TD]Sorcerer 3/Warlock 5[/TD]
[TD]4 1st, 2 2nd, 6 3rd[/TD]
[/TR]
[TR]
[TD]9[/TD]
[TD]Sorcerer 9[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 3 4th, 1 5th[/TD]
[TD]Sorcerer 4/Warlock 5[/TD]
[TD]4 1st, 3 2nd, 6 3rd[/TD]
[/TR]
[TR]
[TD]10[/TD]
[TD]Sorcerer 10[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 3 4th, 2 5th[/TD]
[TD]Sorcerer 5/Warlock 5[/TD]
[TD]4 1st, 3 2nd, 8 3rd[/TD]
[/TR]
[TR]
[TD]11[/TD]
[TD]Sorcerer 11[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 3 4th, 2 5th, 1 6th[/TD]
[TD]Sorcerer 6/Warlock 5[/TD]
[TD]4 1st, 3 2nd, 9 3rd[/TD]
[/TR]
[TR]
[TD]12[/TD]
[TD]Sorcerer 12[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 3 4th, 2 5th, 1 6th[/TD]
[TD]Sorcerer 7/Warlock 5[/TD]
[TD]4 1st, 3 2nd, 9 3rd, 1 4th[/TD]
[/TR]
[TR]
[TD]13[/TD]
[TD]Sorcerer 13[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 3 4th, 2 5th, 1 6th, 1 7th[/TD]
[TD]Sorcerer 8/Warlock 5[/TD]
[TD]4 1st, 3 2nd, 9 3rd, 2 4th[/TD]
[/TR]
[TR]
[TD]14[/TD]
[TD]Sorcerer 14[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 3 4th, 2 5th, 1 6th, 1 7th[/TD]
[TD]Sorcerer 9/Warlock 5[/TD]
[TD]4 1st, 3 2nd, 9 3rd, 3 4th, 1 5th[/TD]
[/TR]
[TR]
[TD]15[/TD]
[TD]Sorcerer 15[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 3 4th, 2 5th, 1 6th, 1 7th, 1 8th[/TD]
[TD]Sorcerer 10/Warlock 5[/TD]
[TD]4 1st, 3 2nd, 9 3rd, 3 4th, 2 5th[/TD]
[/TR]
[TR]
[TD]16[/TD]
[TD]Sorcerer 16[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 3 4th, 2 5th, 1 6th, 1 7th, 1 8th[/TD]
[TD]Sorcerer 11/Warlock 5[/TD]
[TD]4 1st, 3 2nd, 9 3rd, 3 4th, 2 5th, 1 6th[/TD]
[/TR]
[TR]
[TD]17[/TD]
[TD]Sorcerer 17[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 3 4th, 2 5th, 1 6th, 1 7th, 1 8th, 1 9th[/TD]
[TD]Sorcerer 12/Warlock 5[/TD]
[TD]4 1st, 3 2nd, 9 3rd, 3 4th, 2 5th, 1 6th[/TD]
[/TR]
[TR]
[TD]18[/TD]
[TD]Sorcerer 18[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 3 4th, 3 5th, 1 6th, 1 7th, 1 8th, 1 9th[/TD]
[TD]Sorcerer 13/Warlock 5[/TD]
[TD]4 1st, 3 2nd, 9 3rd, 3 4th, 2 5th, 1 6th, 1 7th[/TD]
[/TR]
[TR]
[TD]19[/TD]
[TD]Sorcerer 19[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 3 4th, 3 5th, 2 6th, 1 7th, 1 8th, 1 9th[/TD]
[TD]Sorcerer 14/Warlock 5[/TD]
[TD]4 1st, 3 2nd, 9 3rd, 3 4th, 2 5th, 1 6th, 1 7th[/TD]
[/TR]
[TR]
[TD]20[/TD]
[TD]Sorcerer 20[/TD]
[TD]4 1st, 3 2nd, 3 3rd, 3 4th, 3 5th, 2 6th, 2 7th, 1 8th, 1 9th[/TD]
[TD]Sorcerer 15/Warlock 5[/TD]
[TD]4 1st, 3 2nd, 9 3rd, 3 4th, 2 5th, 1 6th, 1 7th, 1 8th[/TD]
[/TR]
[/TABLE]
Honestly my bias is always towards pure Warlock dipping into others, so that may colour things.