from adding a catback exhaust (2 and 1/2 inch piping) and a stainless steel ceramic coated racing headers (1.75 in and 2.25 out 4-1-1)...
You will get about 4-6 hp per part, maybe a bit more depending on how well they work together 8-15 total. Some parts interact better than others based on flow characteristics etc. The best thing to do is buy from the same manufacturer for both parts if possible, provided that they are a reputable manufacturer. You can also expect to lose 3-6% fuel economy with these new parts.
Header Stainless Steel Piping T201 45° 3.5"16Gauge Mazda Toyota Scion TC Acura , 03 04 05 06 07 SCION XB/XA STAINLESS JDM EXHAUST HEADER, STAINLESS STEEL RACING EXHAUST HEADER 02-06 SCION XA/XB JDM TOYOTA BB IST 1.5L, STAINLESS STEEL RACING EXHAUST HEADER 02-06 SCION XA/XB JDM TOYOTA BB IST 1NZ-FE, DC SPORTS RACE HEADER POLISH STAINLESS STEEL 05-10 SCION tC 2.4L ONE-PIECE 4-2-1, STAINLESS RACING MANIFOLD HEADER/JDM EXHAUST 04-07 SCION TOYOTA xA xB WAGON/BASE, Megan Stainless Steel Header Scion tC 04 05 06 07 08, Megan Stainless Steel Header Scion xA xB 04 05 06 07, SCION xAxB 02 03 04 05 06 07 HP RACING STAINLESS HEADER, STAINLESS RACING MANIFOLD HEADER/EXHAUST 04-07 SCION TOYOTA xA xB WAGON HB/BASE, MEGAN 4-1 1pc Stainless Exhaust Header Scion 04-06 xA NCP61 xB NCP31 1NZ-FE, Megan Stainless Steel Header Scion xA xB 04 05 06 07, Scion TC Stainless steel T3/T4 turbo exhaust header manifold, Megan Racing MR-SSH-STC05 Scion Stainless Steel Header, Stainless Steel Exhaust Racing Header 2003-2007 04 05 06 Scion xB bB xA, MEGAN RACING STAINLESS HEADER Scion xA xB 04-06, MEGAN RACING EXHAUST HEADER STAINLESS FITS 04-08 SCION TC W/O CAT 4-1 1-PIECE, DC SPORTS RACE HEADER POLISH STAINLESS STEEL 08-11 SCION xB 2.4L ONE-PIECE 4-2-1, OBX Stainless Steel Header Manifold Exhaust 03-05 Scion xA 02-04 Toyota Echo 1.5, MEGAN RACING EXHAUST HEADER STAINLESS STEEL FITS 04-06 SCION XA XB 4-1, Stainless Steel Racing Exhaust Header for Scion 2005-2008 tC 2.4L L4 06 07, Stainless Steel Exhaust Header + Downpipe S Pipe Combo for 05 06 07 08 Scion tC, 2008-2010 SCION XB 2.4L 4-1 DOHC STAINLESS STEEL HEADER, OBX Stainless Exhaust Header 03-06 Toyota Scion xA xB, Megan Racing Stainless Header 04-06 Scion xA xB, Megan Racing Stainless Header 04-08 Scion tC, Scion 03-07 xB xA BB Stainless Steel Exhaust Header , TOYOTA SCION XB 08 09 10 RACING STAINLESS STEEL HEADER , 2008-2010 SCION XB 2.4L RACING STAINLESS STEEL HEADER, 08 09 10 TOYOTA SCION XB RACING STAINLESS STEEL HEADER,