are shumacker headers restrictive?

No. In the way you asked the question, Schumacher headers are not restrictive. You gave no comparison, so none can be made. Are they restrictive compared to stock or HP manifolds? No. Are they restrictive compared to Blackjack headers? No. Are they restrictive compared to TTI or other high end headers? The ONLY people who can answer that are those who have SEEN dyno results or those who have run BOTH at the strip and have a basis of comparison. Any other answer is complete speculation and BS because every engine responds differently. Some may make more power with a stepped header. You never know until the dyno or strip. PERIOD.