Let's begin by defining some static values.
The table below shows the cost of making a tiered Classification 4 equipment both from Tier 0 and from the previous tier. These values come from assuming a supplementary Classification 4 equipment of 5kk + the cost of Convergence Fusion (the most common method for tiering up very expensive/bis equipment).

The table below shows the values of all effects.

Below is the table with the increase for all tiers in all effects:

All in all, we have that:
- Upgrading Onslaught from T1 to T2 costs 175,000,000 and gives a 0.55% increase.
- Upgrading Ruse from T1 to T2 costs 175,000,000 and gives a 0.53% increase.
- Upgrading Momentum from T1 to T2 costs 175,000,000 and gives 2.05% increase.
- Upgrading Transcendence from T1 to T2 costs 175,000,000 and gives 0.14% increase.
Given the above, we can say that, if you have a T1 set, 0.55% more in Onslaught is worth 175,000,000 gold.
In this context, upgrading the boots from T0 to T1 would yield the following:
- 0.0125% more in Onslaught, worth 3,977,272.72 gold.
- 0.0125% more Ruse, worth 4,127,358.49 gold.
- 0.05% more Momentum, worth 4,268,292.68 gold.
- 0.00325% more Transcendence, worth 4,062,500 gold.
Therefore, the entire gold value of adding a T1 boots on top of a T1 set is: 3,977,272.72 + 4,127,358.49 + 4,268,292.68 + 4,062,500 = 16,435,423.89.
Below is the table for how much is the gold value of upgrading boots on top of each tiered set, truncating decimals:

The asterisk marks where the gold value of tiering up the boots is higher than the cost of doing so.
Below is a chart with the values from the table above. The blue lines are set to the asterisk marked values.

Notes:
1. We don't care about the T0 set because adding a tiered boots to a T0 set has no effect.
2. We don't care about the T10 set because at that point we have already upgraded the boots to T10.
Conclusion
Assuming you're going to tier up the entire set, the optimal plan is:
1. Upgrade set T0->T1->T2
2. Upgrade boots T0->T1
3. Upgrade set T2->T3
4. Upgrade boots T1->T2
5. Upgrade set T3->T4
6. Upgrade boots T2->T3->T4
7. Upgrade set T4->T5
8. Upgrade boots T4->T5
9. Upgrade set T5->T6
10. Upgrade boots T5->T6->T7
11. Upgrade set T6->T7
12. Upgrade boots T7->T8
13. Upgrade set T7->T8
14. Upgrade boots T8->T9
15. Upgrade set T8->T9
16. Upgrade boots T9->T10
17. Upgrade set T9->T10