If you're using spell slots a fairer comparison is still RotD plus spiritual weapon vs maximised whatever. Both cases are using spellslot plus channel divinity.
As requested...
Assuming 7th level:
Light Cleric
Round 1 RotD plus SW upcast into 4th with 20 WIS: 2d10+7 + 2d8+5
(32 average)
Round 2 RotD plus SW: 2d10+7 + 2d8+5
(32 average)
Rounds 3+ SW: 2d8+5
(14 average)
Tempest Cleric
Round 1 Upcast into 4th level Call Lightning, with Maximized strike:
(40)
Round 2 Maximized Lightning strike:
(40)
Rounds 3+ Non-max Lightning strikes: 4d10
(18 average)
Both use one 4th level slot and 2x Call Divinities.
Tempest is doing 4-8 damage more per round on average. And has the ability to continue to potentially hit multiple targets in Rounds 3+, whereas Light is limited to damaging one target at a time in Rounds 3+.
If we continue the same routine to 9th level and upcast Call Lightning/Spiritual Weapon into a 5th level slot, Tempest starts doing 16-18.5 more damage per round on average than Light (34/34/14 vs. 50/50/22.5).
Now, I know you specified that specific apples to apples comparison of "RotD plus Spiritual Weapon vs Maximised Whatever". But to be truly fair, in the bigger picture, there are a number of different qualifiers:
-Not every SW attack is going to hit, and not every monster targeted is going to fail their DEX saves against CL/RotD. And there will be situations in which the wider range of RotD will allow you to catch enough additional enemies than the smaller radius of CL that you could be doing more damage overall in some rounds. Someone much smarter than me, with way more time than me, might be able to create a spreadsheet to crunch the numbers of every situation at every level against every number of enemies with every AC and DEX save bonus, to truly declare using precise mathematics which Cleric subclass is the better blaster in every specific scenario.
-In Rounds 3+, CL requires your Concentration and your Action to call down, but leaves your Bonus Action free. Whereas SW requires your Bonus Action to attack, leaving your Action and Concentration free. So if we're talking real world, and not just "2x CD and 1x 4th/5th spell only", the gap would be smaller from the Light cleric tossing out an attack cantrip or additional spell with their Action in Rounds 3+ for more points of potential damage per round. The flip side is that the Tempest Cleric's Bonus Action is still free for things like Healing Words, or casting a Spiritual Weapon of their own starting in Round 2.
-As you correctly pointed out, Call Lightning is not usable everywhere. That's really the only thing keeping the Tempest Cleric from becoming definitively overpowered. If Tempest is relying on just one Maximized Upcast Shatter, Light pulls noticeably ahead, with 32/32/14 vs. 40/0/0, or 34/34/14 vs. 50/0/0. Luckily, by Tier 2 and later, Clerics have enough spell slots to get them reliably through the typical adventuring day while slinging plenty of spells throughout combats. They don't have to ration their spells to just one per fight, like this specific comparison you requested.
TL;DR: I stand by my position that Light is better at Tier 1 when spell slots are more precious, while Tempest is better overall at higher levels starting with Tier 2, with the caveat that Light can potentially still be better in some situations. And in a strict comparison as requested of "RotD plus Spiritual Weapon vs Maximised Whatever", Tempest is doing 4-8 or 16-18.5 more damage per round at Levels 7 and 9.