Welsummers are slightly lighter than RIR, so the crosses should theoretically be slightly less productive in terms of meat.
However when crossed with another heavy breed like the Indian game or dorking, you will still get a good amount of meat. But that really depends on what you consider to...
Any 2 DP birds crossed with each other will give you decent meat birds. At this stage the strain of bird you have will be far more important than breed.
For some of the best crosses:
Indian Game (Cornish) x Light Sussex
Indian Game (Cornish) x RIR
RIR x Light Sussex
Wyandotte x Sussex...