PA44 PLT IS REFUSED A LOWER ALT BECAUSE OF ACFT PERFORMANCE AND TURB.

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AFTER RECEIVING OUR CLRNC FROM ELP TO IWA; WE DEPARTED ELP AND CLBED UP TO 10000 FT. ATC ASKED IF WE WERE ABLE TO ACCEPT 12000 FT. I ASKED THE PIC IF HE WANTED TO ACCEPT 12000 FT AND HE SAID OKAY. I NOTIFIED ATC THAT WE WERE OUT OF 10000 FT FOR 12000 FT. OUR CLB UP TO 12000 FT WAS MUCH SLOWER THAN EXPECTED AND UPON REACHING 11700 FT WE WERE NO LONGER ABLE TO CLB. THE PLANE WAS PITCHED UP TO VY BUT WE WEREN'T CLBING. SINCE I WAS WORKING THE RADIOS; I NOTIFIED ATC THAT WE WERE NO LONGER CLBING AND WE NEEDED TO MAINTAIN A LOWER ALT. NORMALLY OUR PIPER SEMINOLE CAN CLB TO AND CRUISE AT 12000 FT WITHOUT ANY PROBS; BUT TODAY IT WAS WARM AND THERE WERE EXTREMELY STRONG WINDS FROM THE W AND THE PLANE WAS NOT PERFORMING AS IT DOES NORMALLY. AFTER WE ADVISED ATC THAT WE WEREN'T ABLE TO CLB TO 12000 FT; THEY TOLD US TO MAINTAIN 11000 FT. WE ASKED IF IT WOULD BE POSSIBLE TO RETURN TO 10000 FT; BUT ATC SAID NO DUE TO TFC. WE CONTINUED FLYING AT 11000 FT; BUT AS WE PASSED OVER THE MOUNTAINS TO THE W OF EL PASO; WE BEGAN TO EXPERIENCE MODERATE TURB AND VERY STRONG DOWNDRAFTS AND UPDRAFTS. WE HEARD OTHER PLANES IN THE AREA REQUESTING DIFFERENT ALTS AND WE HEARD OTHER PLTS TELL ATC THAT THEY WERE 'TRYING TO MAINTAIN 11000 FT.' WE CONTINUED TO GET BUMPED AROUND AND AT ONE POINT WE WERE PUSHED DOWN ABOUT 300 FT AND WERE HAVING DIFFICULTY CLBING BACK TO 11000 FT DUE TO THE STRONG DOWNDRAFTS. I QUERIED ATC AGAIN TO SEE IF IT WOULD BE POSSIBLE TO RETURN TO 10000 FT WHERE THE RIDE HAD BEEN MUCH BETTER. HE SAID NO BECAUSE THERE WAS A C172 AHEAD ON THE SAME AIRWAY AT 10000 FT. ATC THEN ASKED 'ARE YOU DECLARING AN EMER?' I ASKED THE PIC IF HE WANTED TO DECLARE AN EMER AND HE SAID 'NO.' I TOLD ATC THAT WE WERE NOT DECLARING AN EMER; BUT WE WOULD LIKE TO GET DOWN TO 10000 FT ASAP. APPROX 2 MINS LATER; ATC TOLD US TO TURN R HDG 345 DEGS FOR SPACING. WE COMPLIED. ATC THEN GAVE US A TURN BACK TO THE L AND TOLD US TO INTERCEPT THE VICTOR AIRWAY; SLOW TO 110 KTS OR LESS. WE COMPLIED; INTERCEPTED THE COURSE; AND REMAINED BELOW 110 KTS OR LESS. ATC ALSO INSTRUCTED US TO MAINTAIN 10000 FT. WE STAYED AT 10000 FT AND 110 KTS OR LESS FOR ABOUT 100 MI. WHEN ATC HANDED US OFF TO THE NEXT CTR; HE TOLD US TO CONTACT ZAB WHEN WE LANDED TO DISCUSS OUR DEV. WHEN WE CALLED ZAB UPON LNDG; THE PLT WAS TOLD THAT THERE WAS A 'SEPARATION ISSUE' BTWN OUR PLANE AND A C172 AHEAD OF US. WE NEVER SAW THE OTHER PLANE AND WERE UNAWARE OF ANY SEPARATION PROBS. LOOKING BACK AT THE SIT; THE PLT AND I WONDERED WHY WE HADN'T BEEN VECTORED AROUND THE CESSNA EARLIER. WE ALSO SHOULD HAVE CONSIDERED THE PROBS WE WERE GOING TO HAVE AT A HIGHER ALT DUE TO THE STRONG HEADWINDS. IN THE FUTURE; I WOULD NOT HESITATE TO DECLARE AN EMER IF THE MOUNTAIN WAVES WERE STRONG ENOUGH TO MAKE IT NEARLY IMPOSSIBLE TO MAINTAIN MY ALT.