Here's the response from P&J. Thought I'd pass it on . . .

Jack

Through our lab testing we have found that the tube axle flexes more than anyone in our industry ever imagined. Not as radical as an I-beam but it does flex non the less. We have also found that the hairpins vs. original solid wishbones carries a tremendous amount of flex. The taboo combination is tube axle and stock Ford wishbones. Although I have seen this done, but we do not recommend it.

Now back to your original question, this has been used in several vehicles in recent years. No stress related problems at all. So with lab tests, vehicle tests, and overall knowledge and experience with hairpins/4-BARS and tube axles our opinion this system is fine.

Thanks,

Jason