ALT OVERSHOOT IN TEXAS ACCOUNT LATE ALT ASSIGNMENT AND A HIGH RATE OF CLB.

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ON CLBING OUT VFR FROM 4R2 ARPT; I CALLED CTR (ZHU) TO GET A 'POP-UP' IFR CLRNC. I CALLED AT 5000 FT AND NO RESPONSE. I AGAIN CALLED AT 5800 FT AND GOT AN ANSWER OF 'MAINTAIN 6000 FT' AND A SQUAWK CODE. MY PRESENT CLB RATE WAS +2500 FPM. I GAVE A READBACK AND BEGAN TO LEVEL OFF BUT HAD TO DSND OUT OF 6400 FT BY THAT TIME. I SET THE SQUAWK CODE WHILE LEVELING. CTR CALLED AND SAID 'I SEE YOUR OUT OF 6000 FT; CLB AND MAINTAIN 14000 FT AND FLY DIRECT UUA.' I DID. I DID NOT EXPECT SUCH A SUDDEN ALT LIMIT. I WAS CLBING QUICKLY AS A MATTER OF PROC TO CLB UP AWAY FROM LOWER LEVEL TFC AND TURB. TO HAVE LEVELED OFF SO QUICKLY WOULD SURELY HAVE RESULTED IN A NEGATIVE 'G' LOAD (FLOATING PAX) SO I DID RETURN TO 6000 FT AS QUICKLY AS WAS REASONABLY POSSIBLE. I FEEL THAT THE 6000 FT ALT RESTR WAS TO KEEP ME BELOW TFC FLYING INTO AUSTIN; TX; UNTIL MY POS COULD BE LOCATED ON RADAR. THIS ALTDEV WAS QUICK; MINOR AND SEEMED A NON EVENT. HOWEVER; IT WAS A DEV THAT WAS UNAVOIDABLE. ALSO; I WAS VFR AT THAT TIME SINCE I HAD YET TO RECEIVE A CLRNC. NO OTHER ACFT WAS INVOLVED. I AM NOT SURE WHAT ATC USES AS A RULE FOR LEAD TIME ON AN ALT ASSIGNMENT BUT THIS ONE WAS A LITTLE TIGHT.