It depends on which transition
Not all CLB transitions are equal. CLB 6→7 and CLB 8→9 each add +32 direct language points (for all four modules). CLB 7→8 adds +24, and CLB 9→10 adds only +12. The two 'golden jumps' - 6→7 and 8→9 - are the highest-value transitions.
Transferability makes CLB 9 a threshold
CLB 9 is special not just for direct points but because it doubles transferability bonuses. If you have post-secondary education or foreign work experience, reaching CLB 9 on all four skills can unlock 25–100 extra CRS points beyond the language column.
Is 4–8 weeks of prep worth it?
Realistically, moving one CLB level (e.g., CLB 7 → CLB 8 in a weaker module) takes 4–8 weeks of focused prep. That can add 24–32 direct CRS points. In context, that's more CRS than a second year of Canadian work experience (+13 pts). For most Express Entry candidates, yes - it is worth it.