It would seem to me that the ultimate cross to improve the type of BLRW's would not be white (either recessive - or dominant) but the best option might well be Silver Laced birds.
I say this for a number of reasons.
One, (and it's huge) is that you are already dealing with laced birds. lacing is a complex set of genes - and going away from laced birds will take many extra years to get back to good true-breeding lacing.
Second, Silver Laced birds with good type are present and available.
Thirdly, White birds have a softer feather, and soft feathering isn't necessarily the best for clearly defined lacing.
Fourth, the lack of the Mahogany gene - is an over-rated problem. For one thing, being a dominant gene, only a single copy of the Mahogany gene need be present for it to express itself.That means, a cross-bred BLRW (of good ground colour) and a SLW will throw young that will show the Mahogany gene in the F1 generation. In the F2 generation, one will get back to 1/4 homozygous (double-dose) Mahogany birds, 1/2 of the birds will be heterozygous Mahogany birds (single-dose - but still showing the effect) and only 1/4 will be birds without the Mahogany gene - therefore easily sorted out and culled, if that is desired.
Therefore, I think using the best type SLW's as a means of improving Wyandotte type in BLRW's, seems to me, to be a better option than using white birds. But, if good GLW's can be found - they'd work well also.
