I have just transferred 2250 MR points from a US AMEX to IB+. The Amex webpage quoted a promotion, giving 250 MR = 250 Avios (rather than the new standard 250 MR = 200 Avios). However, when the points were transferred I got a 25 % extra:
2/12/15 Promocion 25% Transferencia Avios Mrb Amex Financiero 563
2/12/15 Transferencia De Membership Rewards De American Express Financiero 2.250
So 2250 + 563 Avios for 2250 US AMEX MR.