Since soma (carisoprodol) is a prodrug, will drinking white grapefruit juice reduce the efficacy of it, since it uses enzymes to turn it to it's active metabolite?

Not necessarily: It is not clear how much effect of Soma (carisoprodol) is due to Carisoprodol and how much to its active mebolite meprobamate. Grapefruit juice would increase the effect of Carisoprodol even if it does decrease conversion to meprobamate.

No: Certain compounds contained in grapefruit juice, known as furanocoumarins, may impact the cyp3a4 isoform of cytochrome p450. This may result in inhibition of metabolism of certain drugs. Although there are some opioids and sedative/hypnotics whose metabolism by the enterocytes of the small intestine may be affected by grapefruit juice, Soma (carisoprodol) is not one of them.