The fundamental airframe is essentially the same, The engine characteristics are essentially the same. and the models are similar in payload/range characteristics.
By all apparence, the Chinese J-15 now actually reflected less airframe and power plant characteristic improvements compared to the original Su-33 from 30 years ago than versions of many typical 4th generation fighters that came out 20-30 years after the original model shows compared to the original model. For example, the the J-15 is more similar to the su-33 than F-14D is to F-14A, and much more similar than the F-18E is to F-18A.
I agree about F-18 difference, though the F-18E airframe is fundamentally different altogether (Much larger).
I think F-14A and D is a good comparison for differences between Su-33 and J-15. However, the powerplant being the same between Su-33 and J-15 is just a limitation of production and conservative procurement strategy. Changing to a CATOBAR carrier would yield as much benefits as powerplant upgrade in some ways.